Support Forgejo and Gitea as additional source providers

[mmatous]

I use Codeberg (public Forgejo instance) and self-host Forgejo for my projects, so I'd like pydata theme to have first-class support of this forge. I can provide a PR. Looking through the code, it seems fairly straightforward. Additionally Gitea support would be included mostly for free, bc the relevant URLs haven't diverged since Forgejo/Gitea fork.

One small issue may be that Font Awesome doesn't include icons for Codeberg, Forgejo nor Gitea (yet? v7 might include Codeberg, but who knows) but that could be handled either by bundling a svg or leaving it as generic fa-code-branch and letting users supply their own.

[drammock]

+1, PR welcome