Show off quantity_support() in units notebook

Suggestion by @eerovaher - it is really nice that one can make plots that automatically show the units on the labels (and convert different elements). Would be nice to include that in the notebook -- and possibly even in the slides!
See https://docs.astropy.org/en/stable/visualization/matplotlib_integration.html

Start with the linked matplotlib integration documentation, then locate the units notebook and any related workshop slides. Add a demonstration of quantity_support() showing units on plot labels and conversion between elements, and verify that the notebook renders the intended result.

Written by the indexing model from the issue text.