docs: move docs to docusaurus

docs/docs/mdx.md

@@ -0,0 +1,17 @@
+---
+id: mdx
+title: Powered by MDX
+---
+
+You can write JSX and use React components within your Markdown thanks to [MDX](https://mdxjs.com/).
+
+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_!