Force-pushed because I also needed to rename the changenote filename, and I wanted that in the first commit.

Notes on second commit for second reviewer:

For some reason, modifying the artist_api.rst triggered warnings and therefore a Circle-CI failure like this. I demonstrated that removing the inheritance diagram at the top of the file made the problem go away, so that seems to be the cause but I'm unclear why the inheritance diagram has not been a problem before now.

To fix it, I went ahead and added docs for the missing private artists.

For _ImageBase and _AnnotationBase I think these are reasonable to include as they have useful methods that do not show in the docs for the public arists. E.g. get_shape, get_size, set_data on _ImageBase and get/set_annotation_clip on _AnnotationBase.

For _AxesBase I made it as unobtrusive as possible.

I will note that all the doc problems were already listed in doc/missing-references.json

Ah, I didn't know about that file! I had hit the same docs failures after I modified artist_api.rst in #31162, so it's great to now understand why that happened.

I had heard of that file but never made the effort to learn what it's about! Thanks @QuLogic it's good to have an explanation.

I am in two minds what to do about the _AxesBase reference. The stub file I created seems like documentation clutter, but the missing_references.json solution is brittle - it has confused at least three of us. Does anyone have an opinion which solution is the lesser evil?