refactor: stricter linting rules

[FloLimebit]

This branch introduces the following changes:

• Ruff linting rules in pyproject.toml file made explicit and as strict as possible/reasonable
  • each linting rules has a short code comment explaining what the rule is about
  • an exclusion list was added to avoid conflicts with the auto formatter
  • the ruff option preview = true got activated -> ruff supports supports now potentially unsafe fixes.
  • since we have an extremely high test coverage, I'd suggest we go with this option and see if it breaks something down the road
• fix for generating API docs of treatment effect module
• small improvement to docs/conf.py ensuring that __all__ definitions are respected
• Docstrings Developer Guide got extended to be more explicit and explanatory

[JabobKrauskopf]

@MarIniOnz

Also, when performing make lint and make format this error pops up. Is it expected? We should probably fix it before merging:

I don't get this error. Could you try running make clean and then e.g. make format again?