Markdown rendering improvements

[Keats]

In 0.23, the markdown rendering will be significantly rewritten to remove shortcodes (well that's the plan for now). Instead, the content will be optionally rendered by Tera.

There are quite a few issues around footnotes (#2896 , #2765 , #2713 , #2613 , #1286) and around shortcodes. It would be nice to fix everything in one go.

Let me know if there are other big issues around the markdown bits. Once this is in, I think it will be much easier to integrate typst?

Also #2104