Small improvements to canvasagg example.

[anntzer]

In particular, show how Pillow can be used to save images in formats not
directly supported by Matplotlib. Also directly convert the buffer to
an array (with only brief, in-passing mention of memoryviews), as
memoryviews are rather technical.

See https://discourse.matplotlib.org/t/create-a-bmp-from-pyplot/22099.

Perhaps https://matplotlib.org/stable/gallery/misc/agg_buffer.html and https://matplotlib.org/stable/gallery/misc/agg_buffer_to_array.html could be merged into this example too?

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• 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).

[timhoffm]

Anybody can merge after CI pass.