Migrate to gatsby-plugin-mdx v4

[robdy]

Still WIP rewriting MDX to new format

https://mdxjs.com/docs/troubleshooting-mdx/

• [x] Rewrite all <Something\> to \<Something\>
• [ ] Recreate broken CodeBlock component
• [x] Fix header anchors
• [x] Fix inline code being wrapped in <div> - might be related (src)

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