feat(v2): Improve the initial templates (#4302)

* feat: add getting started doc at classic inital templates

* fix: improve the contents of getting started page

* fix: fix slug routing

* fix: rename gettingStarted to getting-started and re-adjust the content

* feat: add markdown-features docs

* feat: add a page on how to create a simple document

* feat: add a page on how to create pages

* feat: add create a post doc

* feat: add thank you page with whats next

* fix: reduce the content of the init templates

* update the init template

Co-authored-by: Lisa Chandra <52909743+lisa761@users.noreply.github.com>
Co-authored-by: Javid <singularity.javid@gmail.com>
Co-authored-by: slorber <lorber.sebastien@gmail.com>

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

@@ -0,0 +1,38 @@
+---
+title: Create a Document
+---
+
+Documents are pages with a **sidebar**, a **previous/next navigation** and many other useful features.
+
+## Create a Document
+
+Create a markdown file at `docs/my-doc.md`:
+
+```mdx title="docs/hello.md"
+---
+title: Hello, World!
+---
+
+## Hello, World!
+
+This is your first document in **Docusaurus**, Congratulations!
+```
+
+A new document is now available at `http://localhost:3000/docs/hello`.
+
+## Add your document to the sidebar
+
+Add `hello` to the `sidebars.js` file:
+
+```diff title="sidebars.js"
+module.exports = {
+  docs: [
+    {
+      type: 'category',
+      label: 'Docusaurus Tutorial',
+-     items: ['getting-started', 'create-a-doc', ...],
++     items: ['getting-started', 'create-a-doc', 'hello', ...],
+    },
+  ],
+};
+```