[MNT] Add numpydoc validation to CI

[RNKuhns]

Describe the maintenance issue

It would be good to add numpydoc validation to the CI routines. Some information is available on the numpydoc validation documentation.

[RNKuhns]

To validate a class on the command line there:

python -m numpydoc numpy.ndarray --validate

[RNKuhns]

Looks like numpy/numpydoc/pull/454 should provide the functionality we want.

[fkiraly]

hm, would it substantially impact the current style of docstrings? I suppose the only way to find out is to run the check...