docs(v2): Reorganize/split the guides doc sections (#3975)

* docs reorg

* refactor docs/markdown features section

* fix broken links after docs refactor

website/docs/guides/markdown-features/markdown-features-react.mdx

@@ -0,0 +1,69 @@
+---
+id: react
+title: Using React
+description: Using the power of React in Docusaurus Markdown documents, thanks to MDX
+slug: /markdown-features/react
+---
+
+import BrowserWindow from '@site/src/components/BrowserWindow';
+
+Docusaurus has built-in support for [MDX](https://mdxjs.com/), which allows you to write JSX within your Markdown files and render them as React components.
+
+:::note
+
+While both `.md` and `.mdx` files are parsed using MDX, some of the syntax are treated slightly differently. For the most accurate parsing and better editor support, we recommend using the `.mdx` extension for files containing MDX syntax.
+
+:::
+
+Try this block here:
+
+```jsx
+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.
+
+I can write **Markdown** alongside my _JSX_!
+```
+
+Notice how it renders both the markup from your React component and the Markdown syntax:
+
+export const Highlight = ({children, color}) => (
+  <span
+    style={{
+      backgroundColor: color,
+      borderRadius: '2px',
+      color: '#fff',
+      padding: '0.2rem',
+    }}>
+    {children}
+  </span>
+);
+
+<BrowserWindow minHeight={240} url="http://localhost:3000">
+
+<Highlight color="#25c2a0">Docusaurus green</Highlight>
+{` `}and <Highlight color="#1877F2">Facebook blue</Highlight> are my favorite colors.
+
+I can write **Markdown** alongside my _JSX_!
+
+</BrowserWindow>
+
+<br />
+
+You can also import your own components defined in other files or third-party components installed via npm! Check out the [MDX docs](https://mdxjs.com/) to see what other fancy stuff you can do with MDX.
+
+:::caution
+
+Since all doc files are parsed using MDX, any HTML is treated as JSX. Therefore, if you need to inline-style a component, follow JSX flavor and provide style objects. This behavior is different from Docusaurus 1. See also [Migrating from v1 to v2](../../migration/migration-manual.md#convert-style-attributes-to-style-objects-in-mdx).
+
+:::