Add module docstring for `experiments` module

[williambdean]

The experiments module getting a docstring will populate here: https://causalpy.readthedocs.io/en/stable/api/index.html

https://github.com/pymc-labs/causalpy/blob/91ecda24c4df3128f4810a3c34f7c5a3f063da07/causalpy/experiments/__init__.py?plain=1#L13