PR Summary

This groups the pyplot functions in logical groups, which should make it easier to find functionality.

To keep things simple this only regroups the existing listed functions (note that the test_doc_pyplot_summary guarantees that no function was accidentally added or removed). One can and should do a bit more (see notes below), but I'll leave that for a follow-up PR.

Notes:

• There have been and still are some module-level variables missing, most prominently rcParams.
• I've copied the original "Colormaps" section verbatim to "Colormapping-related commands". This is a bit out-of-style because all other sections only contain the autosummary links. However two aspects make this non-trivial:
  1. the variables colormaps and color_sequences do not have autogenerated stubs. It seems autosummary_generate does not pick them up. So we need explicit .. autodata:: for them somewhere.
  2. we have two text paragraphs that currently have no other place to live.