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/create-a-page.md

@@ -0,0 +1,45 @@
+---
+title: Create a Page
+---
+
+Any React or Markdown file created under `src/pages` directory is converted into a website page:
+
+- `src/pages/index.js` -> `localhost:3000/`
+- `src/pages/foo.md` -> `localhost:3000/foo`
+- `src/pages/foo/bar.js` -> `localhost:3000/foo/bar`
+
+## Create a 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';
+
+function HelloWorld() {
+  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 a Markdown Page
+
+Create a file at `src/pages/my-markdown-page.md`:
+
+```mdx title="src/pages/my-markdown-page.md"
+---
+title: My Markdown page
+---
+
+# My Markdown page
+
+This is a Markdown page
+```
+
+A new page is now available at `http://localhost:3000/my-markdown-page`.