docs: add docstrings across the codebase

[refeed]

Most functions in the project still lack docstrings. Contributions welcome — a good starting point
is src/tirith/core/ and each provider's handler.py.

Rules for contributing

1. Use the reST docstring format:

   """
   Short description.
   
   :param param1: what it is
   :returns: what is returned
   :raises KeyError: when it raises
   """
   

2. One file per PR, so contributors don't collide on the same file.
3. Mention this issue in your PR description (e.g. Related to #<this issue>).
4. PR/commit title format: <name of the file>.py: Add docstring.