Test for warnings in documentation builds

[MicahGale]

When I was building the sphinx documentation locally with make html I noticed quite a few warnings were being issued, which usually means the documentation will be there, but the formatting will be wrong. You can make these fatal errors for Sphinx if you want to set the standard that formatting errors aren't allowed with:

sphinx-build <source> <output> -W --keep-going -E

This is what we do in MontePy's CI.

[clemekay]

Thanks Micah, that's useful to know!