For "Documentation is sphinx and numpydoc compliant" is there a way I can verify that? Do the circle CI runs check that? Maybe there will be a build of the docs I could look at to make sure it looks correct?

You can check the output in the CircleCI logs for the location of the built documentation, follow that link and see that it looks OK.

The documentation build of your PR is available at https://circleci.com/gh/matplotlib/matplotlib/13683/artifacts/0//home/circleci/project/doc/build/html/index.html You find this link via he "Details"-Button on the checks.

This is only a minor point, but in general the documentation mostly uses a passive style, like "A parameter can be set." instead of "You may set a parameter."
I don't think this is relevant for the tutorial, where "you" is also used at other places, but for the docstring itself?

Should I change to passive? I thought it may be but I did a ctrl-f "you" in https://matplotlib.org/users/legend_guide.html and found examples so I thought it'd be fine.

My vote is for active voice most of the time.

Nevermind, even the Axes.legend docstring addressed the reader in person before this change.