Port to remark?

[agentofuser]

Just found out about https://github.com/jake-low/remark-sectionize, which might have made this unnecessary 🤦‍♂️

In my defense, there's the classnames and stuff. Maybe contribute remark code upstream to remark-sectionize.

[nkuehn]

Opinion: I don't see the rehype-based implementation as unnecessary. E.g. when using MDX, not all headings are generated on markdown-level, some could be the result of rendering a react component.
Update: Does not work since the react components are mdxhast and thereforce not touched by plugins working on hast