Find a way to include favicon by default in theme

[astrofrog]

At the moment, the favicon name has to be set in the conf.py, but it might be possible to configure the theme so that the astropy favicon is used.

[kakirastern]

Hi @astrofrog, after working on the Astropy project menubar recently, I might have gained some knowledge to be of some use here... Is the issue still open, and if so how can I help? (Some suggestions of approach would be much appreciated.)

[pllim]

@astrofrog is this still relevant with https://github.com/astropy/astropy-sphinx-theme/blob/main/astropy_sphinx_theme/astropy_unified.py ?