Revisit numpydoc docstring validation

[duposyl]

#44 turned off the numpydoc validation and warnings as it was slowing down the doc build significantly (from a few minutes to a few hours). We need to revisit the issue at some point, and see if we can get that to work properly.

[jorgepiloto]

Version 1.6.0 for numpydoc includes a pre-commit hook. This decouples docstring validation from the documentation build process.