Documentation - Suggestions

I started to work on the documentation (see #41) and I have a few suggestions:

- Include source-code documentation into sphinx
- Provide the documentation in both german and english
- Rethink the README - currently it is a mix of quickstart, documentation for specific apis and links to more documentation on autogenerated api-clients; IMO it should only contain a short description of the repo, how to install it and where to find more documentation
- Provide an Issue template for documentation related issues and tag them as _documentation_, which would make it easier to plan and track documentation progress and allow to filter bugs and other issues.
- Host the documentation on [readthedocs](https://readthedocs.io) (@LilithWittmann thats something you should [look into](https://docs.readthedocs.io/en/stable/tutorial/))