Use Sqhinx docs instead of mkdocs

[michaeldeistler]

This changes our documentation from mkdocs to sphinx. This has two benefits:

1. it looks more professional (see screenshot below)
2. we can host the documentation on readthedocs.

I moved the old documentation to mkdocs and will remove it in a future PR (I just want to keep it for now such that we can still use the old website in case anything goes wrong with readthedocs).