RTD template: add project's github link to every docs page

[hombit]

It could be a GitHub logo in the top right corner.

[hombit]

Prototyped in astronomy-commons/hats#498, but with a different HTTP theme

[delucchi-cmu]

Or put a link in the left-hand-side navigation panel, a la tinygp:

https://tinygp.readthedocs.io/en/stable/index.html

But generally agree with making it easier to find the github link. I've been going nuts looking at other open source projects and just wanting to get to their source code!!