Switch documentation to mkdocs + mkdocs material

[schlegelp]

This is a pain in the neck but I think migrating to mkdocs would pay off in the long run:

1. The build process is so much smoother and faster than Sphinx + RTD (live preview!)
2. Much more modern look and feel
3. API documentation (via mkdocstrings) is just incredible
4. Markdown instead of freaking RST

and the lost goes on....

As an example, check out the docs for Octarine I cobbled together over a weekend.