Add sphinx-favicon

[Timo Cornelius Metzger]

I recently created this Sphinx extension that allows you to add custom favicons. With sphinx-favicon, you can define favicons directly in conf.py, with different rel attributes such as "icon" or "apple-touch-icon" and any favicon size.

@astrojuanlu has suggested I mention this extension here - I'd love to see it included in your list of Sphinx extensions!

[Manuel Kaufmann]

Hi @tcmetzger! Would you like to open a PR to add this extension following the current structure the others are following? 💪🏼