Two more accessibility suggestions

[Oscar Levin]

Dave Kung relayed some suggestions to me on slack:

1. We have multiple navigation panels (top bar, and table of contents, maybe also the footer prev/up/next?).  These are all labeled as navigation, but it would be better to be more descriptive (Table of Contents, Navigation bar).

2. A best practice for typesetting math that contains plain text is to include the plain text inside math mode, so the entire equation is grouped as "one idea".  Perhaps we could include this in the guide.