set up docs based on docstrings and publish HTML versions

Now that fdroidserver has a stable API for some pieces, there should be HTML docs published. It seems that this should be done in the most Pythonic way possible. That seems to be by using sphinx to generate docs based on the docstrings. The Numpy docstring format looks best for our needs, IMHO, based on a quick glance.

• set up HTML doc generation based on docstring (e.g. sphinx)
• publish docs (e.g. readthedocs.org, github pages, gitlab pages, etc) -> published here
• convert docstrings to standard format (e.g. Numpy)