DOC: more pcolor fixes

[jklymak]

PR Summary

Closes #16493

Note I've removed a bunch of "user_demo" examples. I made those back when I made the original tutorials largely because I thought I was supposed to, but I think we want to not duplicate tutorials and examples, so I have removed them.

From #16493, These remaining files are meant to have warnings emitted:

• matplotlib/examples/images_contours_and_fields/pcolormesh_grids.py:66
• matplotlib/examples/images_contours_and_fields/pcolormesh_levels.py:67

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[anntzer]

Perhaps just set the rcParam instead?
Also needs a rebase.

[jklymak]

I don't think there is anything wrong with being explicit here, even if we plan to change the default. I am not a particular fan of rcParams in examples because the user doesn't know what the rcParam value is.

[anntzer]

I agree with being explicit but the examples here are more about different normalizations, not about edge/shading behavior, so I think they slightly distract on the main message. OTOH I won't insist on the change either.
Waiting for CI.

[QuLogic]

Unfortunately needs a rebase again.

[jklymak]

... rebased