Add missing docstrings

[LucaMarconato]

Some of the members and attributes that were previously documented thanks to the code that got removed here: https://github.com/scverse/spatialdata/pull/830/files, need a docstring. In particular methods of BaseTransformation, like to_affine_matrix(), need to be documented.

[Taimoor092]

Hi, I'd like to work on this! I'll start by adding docstrings to BaseTransformation methods including to_affine_matrix(). I'll look at the current codebase and open a PR once I have a draft ready.