Infrastructure for notebooks in Sphinx docs

With this you can just put notebook files into the documentation and they behave similar to regular md pages. There is no example yet.

One point to discuss: There is a pre-commit check which checks and strips out all outputs from notebooks (so that they are clean in git). I like this approach as it makes for clean MRs. Downside is that you cannot see the full notebook here in GitLab (but the docs pages with full output are just some clicks away).

Added (pre-commit) check on unused imports. Also added black, isort and pyugrade checks to notebooks.