Numerous documentation build warnings

[Zeitsperre]

There are a lot of documentation build warnings (~230 at time of writing).

The majority of these are due to multiple targets being produced (sphinx-autodoc alongside user-generated API documentation; requires that one of these be given the attribute of :noindex:). The remaining ones require a keen eye (small typos in docstrings, formatting issues, etc.), or they require some structural decisions with the documentation (pages/notebooks not indexed in the docs that are either should be ignored or added).