Mostly unused glossary still exists in our docs #18491

Problem

Discussed in the GSOD call, this feels like something that we wanted to add at some point but never really got around to using. It currently just contains a list of backends (mostly) and links out to their respective external pages.

https://matplotlib.org/glossary/index.html?highlight=glossary

Suggested Improvement

After making sure that the places that link to the glossary correctly link out to the appropriate pages, the glossary should probably just be deleted (consensus on call).