feat(v2): auto-generated sidebars, frontmatter-less sites (#4582)

* POC of autogenerated sidebars

* use combine-promises utility lib

* autogenerated sidebar poc working

* Revert "autogenerated sidebar poc working"

This reverts commit c81da980

* POC of auto-generated sidebars for community docs

* update tests

* add initial test suite for autogenerated sidebars + fix some edge cases

* Improve autogen sidebars: strip more number prefixes in folder breadcrumb + slugs

* fix typo!

* Add tests for partially generated sidebars + fix edge cases + extract sidebar generation code

* Ability to read category metadatas file from a file in the category

* fix tests

* change position of API

* ability to extract number prefix

* stable system to enable position frontmatter

* fix tests for autogen sidebar position

* renamings

* restore community sidebars

* rename frontmatter position -> sidebar_position

* make sidebarItemsGenerator fn configurable

* minor changes

* rename dirPath => dirName

* Make the init template use autogenerated sidebars

* fix options

* fix docusaurus site: remove test docs

* add _category_ file to docs pathsToWatch

* add _category_ file to docs pathsToWatch

* tutorial: use sidebar_position instead of file number prefixes

* Adapt Docusaurus tutorial for autogenerated sidebars

* remove slug: /

* polish the homepage template

* rename _category_ sidebar_position to just "position"

* test for custom sidebarItemsGenerator fn

* fix category metadata + add link to report tutorial issues

* fix absolute path breaking tests

* fix absolute path breaking tests

* Add test for floating number sidebar_position

* add sidebarItemsGenerator unit tests

* add processSidebars unit tests

* Fix init template broken links

* windows test

* increase code translations test timeout

* cleanup mockCategoryMetadataFiles after windows test fixed

* update init template positions

* fix windows tests

* fix comment

* Add autogenerated sidebar items documentation + rewrite the full sidebars page doc

* add useful comment

* fix code block title

packages/docusaurus-init/templates/classic/docs/tutorial-basics/_category_.json

@@ -0,0 +1,4 @@
+{
+  "label": "Tutorial - Basics",
+  "position": 2
+}

packages/docusaurus-init/templates/classic/docs/tutorial-basics/congratulations.md

@@ -0,0 +1,21 @@
+---
+sidebar_position: 6
+---
+
+# Congratulations!
+
+You have just learned the **basics of Docusaurus** and made some changes to the **initial template**.
+
+Docusaurus has **much more to offer**!
+
+Have **5 more minutes**? Take a look at **[versioning](../tutorial-extras/manage-docs-versions.md)** and **[i18n](../tutorial-extras/translate-your-site.md)**.
+
+Anything **unclear** or **buggy** in this tutorial? [Please report it!](https://github.com/facebook/docusaurus/discussions/4610)
+
+## What's next?
+
+- Read the [official documentation](https://docusaurus.io/).
+- Add a custom [Design and Layout](https://docusaurus.io/docs/styling-layout)
+- Add a [search bar](https://docusaurus.io/docs/search)
+- Find inspirations in the [Docusaurus showcase](https://docusaurus.io/showcase)
+- Get involved in the [Docusaurus Community](https://docusaurus.io/community/support)

packages/docusaurus-init/templates/classic/docs/tutorial-basics/create-a-blog-post.md

@@ -0,0 +1,29 @@
+---
+sidebar_position: 3
+---
+
+# Create a Blog Post
+
+Docusaurus creates a **page for each blog post**, but also a **blog index page**, a **tag system**, an **RSS** feed...
+
+## Create your first Post
+
+Create a file at `blog/2021-02-28-greetings.md`:
+
+```md title="blog/2021-02-28-greetings.md"
+---
+slug: greetings
+title: Greetings!
+author: Steven Hansel
+author_title: Docusaurus Contributor
+author_url: https://github.com/ShinteiMai
+author_image_url: https://github.com/ShinteiMai.png
+tags: [greetings]
+---
+
+Congratulations, you have made your first post!
+
+Feel free to play around and edit this post as much you like.
+```
+
+A new blog post is now available at `http://localhost:3000/blog/greetings`.

packages/docusaurus-init/templates/classic/docs/tutorial-basics/create-a-document.md

@@ -0,0 +1,56 @@
+---
+sidebar_position: 2
+---
+
+# Create a Document
+
+Documents are **groups of pages** connected through:
+
+- a **sidebar**
+- **previous/next navigation**
+- **versioning**
+
+## Create your first Doc
+
+Create a markdown file at `docs/hello.md`:
+
+```md title="docs/hello.md"
+# Hello
+
+This is my **first Docusaurus document**!
+```
+
+A new document is now available at `http://localhost:3000/docs/hello`.
+
+## Configure the Sidebar
+
+Docusaurus automatically **creates a sidebar** from the `docs` folder.
+
+Add metadatas to customize the sidebar label and position:
+
+```diff title="docs/hello.md"
++ ---
++ sidebar_label: "Hi!"
++ sidebar_position: 3
++ ---
+
+
+# Hello
+
+This is my **first Docusaurus document**!
+```
+
+It is also possible to create your sidebar explicitly in `sidebars.js`:
+
+```diff title="sidebars.js"
+module.exports = {
+  tutorialSidebar: [
+    {
+      type: 'category',
+      label: 'Tutorial',
+-     items: [...],
++     items: ['hello'],
+    },
+  ],
+};
+```

packages/docusaurus-init/templates/classic/docs/tutorial-basics/create-a-page.md

@@ -0,0 +1,43 @@
+---
+sidebar_position: 1
+---
+
+# Create a Page
+
+Add **Markdown or React** files to `src/pages` to create a **standalone page**:
+
+- `src/pages/index.js` -> `localhost:3000/`
+- `src/pages/foo.md` -> `localhost:3000/foo`
+- `src/pages/foo/bar.js` -> `localhost:3000/foo/bar`
+
+## Create your first React Page
+
+Create a file at `src/pages/my-react-page.js`:
+
+```jsx title="src/pages/my-react-page.js"
+import React from 'react';
+import Layout from '@theme/Layout';
+
+export default function MyReactPage() {
+  return (
+    <Layout>
+      <h1>My React page</h1>
+      <p>This is a React page</p>
+    </Layout>
+  );
+}
+```
+
+A new page is now available at `http://localhost:3000/my-react-page`.
+
+## Create your first Markdown Page
+
+Create a file at `src/pages/my-markdown-page.md`:
+
+```mdx title="src/pages/my-markdown-page.md"
+# My Markdown page
+
+This is a Markdown page
+```
+
+A new page is now available at `http://localhost:3000/my-markdown-page`.

packages/docusaurus-init/templates/classic/docs/tutorial-basics/deploy-your-site.md

@@ -0,0 +1,31 @@
+---
+sidebar_position: 5
+---
+
+# Deploy your site
+
+Docusaurus is a **static-site-generator** (also called **[Jamstack](https://jamstack.org/)**).
+
+It builds your site as simple **static HTML, JavaScript and CSS files**.
+
+## Build your site
+
+Build your site **for production**:
+
+```bash
+npm run build
+```
+
+The static files are generated in the `build` folder.
+
+## Deploy your site
+
+Test your production build locally:
+
+```bash
+npm run serve
+```
+
+The `build` folder is now served at `http://localhost:3000/`.
+
+You can now deploy the `build` folder **almost anywhere** easily, **for free** or very small cost (read the **[Deployment Guide](https://docusaurus.io/docs/deployment)**).

packages/docusaurus-init/templates/classic/docs/tutorial-basics/markdown-features.mdx

@@ -0,0 +1,144 @@
+---
+sidebar_position: 4
+---
+
+# Markdown Features
+
+Docusaurus supports **[Markdown](https://daringfireball.net/projects/markdown/syntax)** and a few **additional features**.
+
+## Front Matter
+
+Markdown documents have metadata at the top called [Front Matter](https://jekyllrb.com/docs/front-matter/):
+
+```text title="my-doc.md"
+// highlight-start
+---
+id: my-doc-id
+title: My document title
+description: My document description
+slug: /my-custom-url
+---
+// highlight-end
+
+## Markdown heading
+
+Markdown text with [links](./hello.md)
+```
+
+## Links
+
+Regular Markdown links are supported, using url paths or relative file paths.
+
+```md
+Let's see how to [Create a page](/create-a-page).
+```
+
+```md
+Let's see how to [Create a page](./create-a-page.md).
+```
+
+**Result:** Let's see how to [Create a page](./create-a-page.md).
+
+## Images
+
+Regular Markdown images are supported.
+
+Add an image at `static/img/docusaurus.png` and display it in Markdown:
+
+```md
+![Docusaurus logo](/img/docusaurus.png)
+```
+
+![Docusaurus logo](/img/docusaurus.png)
+
+## Code Blocks
+
+Markdown code blocks are supported with Syntax highlighting.
+
+    ```jsx title="src/components/HelloDocusaurus.js"
+    function HelloDocusaurus() {
+        return (
+            <h1>Hello, Docusaurus!</h1>
+        )
+    }
+    ```
+
+```jsx title="src/components/HelloDocusaurus.js"
+function HelloDocusaurus() {
+  return <h1>Hello, Docusaurus!</h1>;
+}
+```
+
+## Admonitions
+
+Docusaurus has a special syntax to create admonitions and callouts:
+
+    :::tip My tip
+
+    Use this awesome feature option
+
+    :::
+
+    :::danger Take care
+
+    This action is dangerous
+
+    :::
+
+:::tip My tip
+
+Use this awesome feature option
+
+:::
+
+:::danger Take care
+
+This action is dangerous
+
+:::
+
+## MDX and React Components
+
+[MDX](https://mdxjs.com/) can make your documentation more **interactive** and allows using any **React components inside Markdown**:
+
+```jsx
+export const Highlight = ({children, color}) => (
+  <span
+    style={{
+      backgroundColor: color,
+      borderRadius: '20px',
+      color: '#fff',
+      padding: '10px',
+      cursor: 'pointer',
+    }}
+    onClick={() => {
+      alert(`You clicked the color ${color} with label ${children}`)
+    }}>
+    {children}
+  </span>
+);
+
+This is <Highlight color="#25c2a0">Docusaurus green</Highlight> !
+
+This is <Highlight color="#1877F2">Facebook blue</Highlight> !
+```
+
+export const Highlight = ({children, color}) => (
+  <span
+    style={{
+      backgroundColor: color,
+      borderRadius: '20px',
+      color: '#fff',
+      padding: '10px',
+      cursor: 'pointer',
+    }}
+    onClick={() => {
+      alert(`You clicked the color ${color} with label ${children}`);
+    }}>
+    {children}
+  </span>
+);
+
+This is <Highlight color="#25c2a0">Docusaurus green</Highlight> !
+
+This is <Highlight color="#1877F2">Facebook blue</Highlight> !