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-extras/manage-docs-versions.md

@@ -0,0 +1,55 @@
+---
+sidebar_position: 1
+---
+
+# Manage Docs Versions
+
+Docusaurus can manage multiple versions of your docs.
+
+## Create a docs version
+
+Release a version 1.0 of your project:
+
+```bash
+npm run docusaurus docs:version 1.0
+```
+
+The `docs` folder is copied into `versioned_docs/version-1.0` and `versions.json` is created.
+
+Your docs now have 2 versions:
+
+- `1.0` at `http://localhost:3000/docs/` for the version 1.0 docs
+- `current` at `http://localhost:3000/docs/next/` for the **upcoming, unreleased docs**
+
+## Add a Version Dropdown
+
+To navigate seamlessly across versions, add a version dropdown.
+
+Modify the `docusaurus.config.js` file:
+
+```js title="docusaurus.config.js"
+module.exports = {
+  themeConfig: {
+    navbar: {
+      items: [
+        // highlight-start
+        {
+          type: 'docsVersionDropdown',
+        },
+        // highlight-end
+      ],
+    },
+  },
+};
+```
+
+The docs version dropdown appears in your navbar:
+
+![Docs Version Dropdown](/img/tutorial/docsVersionDropdown.png)
+
+## Update an existing version
+
+It is possible to edit versioned docs in their respective folder:
+
+- `versioned_docs/version-1.0/hello.md` updates `http://localhost:3000/docs/hello`
+- `docs/hello.md` updates `http://localhost:3000/docs/next/hello`