Release new version of theme

There are a few changes it would be nice to release soon: https://github.com/matplotlib/mpl-sphinx-theme/compare/v3.9.0...main (and would be good to get in https://github.com/matplotlib/mpl-sphinx-theme/pull/106 too), so I propose doing a 3.9.1 (or 3.10.0?) release soon. If there are no objections I will do a new release in a couple of weeks.