To anchor our choices in some concrete examples, I've pushed #125
which creates https://phaustin.github.io/MyST-NB/equno.html from
https://github.com/phaustin/MyST-NB/blob/eqno/docs/examples/eqno_test/equno.md

Questions:

1. do we want or need similar behviour between latex-rendered and html-rendered versions of the same math?
2. What's the best way to implement the behavior we expect from 1)

(prelimnary aside -- @choldgraf -- as the html shows, I'm not getting mystnb.js to actually run on the page and toggle processEnvironments=true -- can you confirm that using the myst environment from eqno_test/environment.yml does properly fomat the no-$$ equation? I see msytnb.js in _build/_static but it doesn't appear to do anything.)