Create UML diagrams

[UML diagrams](https://en.wikipedia.org/wiki/Unified_Modeling_Language) are useful to identify how classes and subclasses relate to each other in OOP, better understand relationships and identify redundancies or unknown links.

Potentially, it could use the same PlantUML extension mentioned in #112, but that one doesn't seem to extract the inheritance tree automatically, which is a pain. There is [an extension for Sphinx using `pyreverse`](https://pypi.org/project/sphinx-pyreverse/), but it does not seem to be maintained or reached a stable state, so probably not an option.