We have so many nice docstrings, but they aren't really rendered
anywhere (expect locally in your IDE), as we did not include the API
docs anywhere.

This change uses sphinx-apidoc to generate autodoc stubs for Sphinx, so
that the docstrings are actually rendered to HTML. This is not perfect
yet (I'm not too happy about the default modules.rst simply listing one
package), but it is good enough for the start and makes it possible to
actually browse the docstrings in a browser.