Implement a TOC extension

[ralsina]

I don't think this is part of any spec but it sure is handy.

If I were to do it (which I may 🤣 ) I would make it behave like python's because I am currently using it:

https://python-markdown.github.io/extensions/toc/

Any relevant standards / alternative specs?

Update: GFM "as used in GitHub" just always adds a TOC

https://github.blog/changelog/2021-04-13-table-of-contents-support-in-markdown-files/