Convert Sphinx docs to mkdocs

The diff is massive, but that's mostly because I deleted docs/build and added that directory to .gitignore 😄 Out of laziness I munged all the docstrings into a single page but I think the original design with one page per module would be better.

The steps to get the docs building is now:

pip install -e '.[docs]'
mkdocs serve

Fixes #8.