Enable footnotes for README.md files

[jyn514]

There's an actual missing feature still, looking at this diff — footnotes. Do you know if these are actually used in Markdown files parsed by docs.rs? They're not supported by GitHub's own README.md renderer, for what it's worth.

Footnotes are now a part of github-favored markdown[^1], is it now possible to add this feature to the docs? [^1]:https://github.blog/changelog/2021-09-30-footnotes-now-supported-in-markdown-fields/

This mostly involves enabling in Comrak, where they're already supported.

Originally posted by @zStruCat in https://github.com/rust-lang/docs.rs/issues/117#issuecomment-1147559768