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.
