The new_manager classmethod may appear to belong more naturally to the
FigureManager class rather than the FigureCanvas class, but putting it
on FigureCanvas has multiple advantages:

• One may want to create managers at times where all one has is a
  FigureCanvas instance, which may not even have a corresponding manager
  (e.g. subplot_tool).
• A given FigureManager class can be associated with many different
  FigureCanvas classes (indeed, FigureManagerQT can manage both
  FigureCanvasQTAgg and FigureCanvasQTCairo and also the mplcairo Qt
  canvas classes), whereas we don't have multiple FigureManager classes
  for a given FigureCanvas class.

Still, put the actual logic in FigureManager.create_with_canvas and
access it via some indirection, as suggested by timhoffm (#22895).

See discussion at #18854 and #22895.

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).