Create automatic docstrings for dims module from original distributions

[ricardoV94]

These currently end up having an empty docstring: https://pymcio--7820.org.readthedocs.build/projects/docs/en/7820/api/dims/generated/pymc.dims.Flat.html. Is it possible to dynamically copy the docstring from the "regular" distribution?

For vector distributions or transforms we already have specific docstrings (and I think we want that there) but scalar ones can probably share the docstring after a search/replace of tensor_like -> xtensor_like

Originally posted by @OriolAbril in https://github.com/pymc-devs/pymc/pull/7820#discussion_r2204912560