fix: add a footer icon for github

[12rambau]

Fix #516

I simply added a github icon in the footer following instructions from the Furo theme: https://pradyunsg.me/furo/customisation/footer/

As mentioned by the autor of the sphinx theme:

Yes, I’m aware that it can be argued that embedding raw HTML in a conf.py file is… ugly. :)

The other alternative would be to use the fontawesome icon lib but you would need to import the full css as well which looks like an overkill for 1 single icon.