Have a working skeleton of Sphinx (HTML) docs

At least the autodoc feature (gathering automatically from the triple-quote comment blocks above classes, methods, etc.) should be working.

And it should be possible to build the docs by either:

• One click in PyCharm (run configuration)
• One simple terminal command (sphinx-build <source_dir> <target_dir>

It should require no extra setup step for a user of this project (because it will be pulled automatically by pipenv together with all the other dependencies in Pipfile).

The task of actually filling the docs (making nice sections, and actually describing things more detailed in prose) is for @rhenning :) So this can be closed as soon as the skeleton is done.