Refactor ruff linting

[FloLimebit]

We need to extend the ruff linting to 1) include docstring linting and 2) further improve code quality.

The following tasks are required:

• add docstring linting and extend contribution guide
• update naming conventions #181
• add further linting rules (check this post for a detailed overview)
• fix linting issues in medrecord submodule #238
• fix docstrings in medrecord submodule #240
• fix linting issues and docstrings in treatment_effect submodule #237