Add some discussion on "Why Matplotlib"

[brian-rose]

Following up on @kitmacleod's comments in #112 and today's EWG meeting discussion, we want to add some text to the Foundations landing page for the Matplotlib section addressing more clearly why one should learn Matplotlib, and maybe how it fits in with some of the other plotting libraries.

[brian-rose]

pinging @jukent, maybe it would be natural to address this while you are working on revisions to matplotlib content in #167 and #170