Add more Markdown backends

[LukeMoll]

Adds the following (CommonMark-compliant) Markdown backends:

• cmark
• commonmark
• mistletoe
• markdown-it-py

At some point in the future, the goal will be to switch to one of these (or another CommonMark implementation). This PR does not intend to change the default backend, but to provide the tools to evaluate a replacement for a future PR to switch to.

TODO

• [x] Update documentation in docs/
  • The abstraction layer barely needs documenting; if someone wants to add docstrings to it they're welcome to.
• [ ] Get syntax highlighting working
  • For the time being, "offline" highlighting (eg with Pygments) is preferred to simply tagging <pre> blocks for a JavaScript-based highlighter to process on the client-side.
  • [x] Mistletoe
    • Working using Pygments
  • [x] CommonMark
    • Working using Pygments
    • No table support
  • [x] Markdown-It
    • Working using Pygments
  • [ ] cmark
    • Unlikely to support syntax highlighting extensions -- this is probably in a "done" state

[sersorrel]

testcase for the various markdown renderers: https://www.hacksoc.org/news/2021-05-25-moving-to-libera.html, specifically does #cs-york render as a heading or not (in a commonmark-compliant renderer, it should not)

[LukeMoll]

That was in fact the article that made me aware of the problem, and I did use it while setting up the new backends 😅

[sersorrel]

i definitely thought it had been brought up somewhere, yeah, but i couldn't find any references to it – i guess maybe it got slacked