CI: adding numpydoc linter

While I'm doing code review I run into a couple of non numpydoc compliant docstrings. Which is not a big deal, but rendering those docstrings won't look good, so we should really fix them up. And to avoid unnecessary nitpicky comments in the review, we may want to do this via CI.

The issue does not name any files, tests, or existing CI entry points. Start by locating the repository's CI configuration and the docstrings that fail numpydoc compliance; done means the identified docstrings render correctly and CI checks for future non-compliance.

Written by the indexing model from the issue text.