This adds two MkDocs extensions to make list handling more flexible.
It took some trial-and-error, but it seems this config works OK.
revsets.md: use saner formatting that is now possible.
sapling-comparison.md: this was the one case I saw made worse by the
new plugins. I changed the Markdown formatting, it still looks sane.
I initially was thinking of using `mdbook`, which looks a little better, but I
think versioning the docs is important, and the features I want are pretty much
only supported by the Mkdocs' "Material" theme.
Mkdocs is written in Python. The prerequesites for building docs on your
machine should be to install Python and Poetry, everything else should be
installed automatically by Poetry. See the edits to `contributing.md` for more details.
