fix(v2): examples should use Node 14 by default on CodeSandbox + regen with alpha72 (#4574)

* codesandbox: use node 14 by default

* Regenerate examples

* Add example json files to prettierrignore

examples/classic/docs/markdown-features.mdx

@@ -0,0 +1,128 @@
+---
+title: Markdown Features
+---
+
+Docusaurus supports the [Markdown](https://daringfireball.net/projects/markdown/syntax) syntax and has some additional features.
+
+## Front Matter
+
+Markdown documents can have associated metadata at the top called [Front Matter](https://jekyllrb.com/docs/front-matter/):
+
+```md
+---
+id: my-doc
+title: My document title
+description: My document description
+sidebar_label: My doc
+---
+
+Markdown content
+```
+
+## Markdown 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).
+```
+
+Let's see how to [Create a page](./create-a-page.md).
+
+## Markdown images
+
+Regular Markdown images are supported.
+
+Add an image at `static/img/docusaurus.png` and use this Markdown declaration:
+
+```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
+
+:::
+
+## React components
+
+Thanks to [MDX](https://mdxjs.com/), you can make your doc more interactive and use React components inside Markdown:
+
+```jsx
+export const Highlight = ({children, color}) => (
+  <span
+    style={{
+      backgroundColor: color,
+      borderRadius: '2px',
+      color: 'red',
+      padding: '0.2rem',
+    }}>
+    {children}
+  </span>
+);
+
+<Highlight color="#25c2a0">Docusaurus green</Highlight> and <Highlight color="#1877F2">Facebook blue</Highlight> are my favorite colors.
+```
+
+export const Highlight = ({children, color}) => (
+  <span
+    style={{
+      backgroundColor: color,
+      borderRadius: '2px',
+      color: '#fff',
+      padding: '0.2rem',
+    }}>
+    {children}
+  </span>
+);
+
+<Highlight color="#25c2a0">Docusaurus green</Highlight> and <Highlight color="#1877F2">
+  Facebook blue
+</Highlight> are my favorite colors.