Add support for Codeberg repositories

[nfrerebeau]

pkgdown only seems to only support GitHub-like links declared in the BugReports or URL fields, but Codeberg's url construction is slightly different.

Would it be possible to add Codeberg support to pkgdown? This requires only a few changes (see here) and, if this suggestion is acceptable, I can submit a PR.

[hadley]

I'm generally sceptical about adding support for tools that none of the core team use, but this seems simple enough that it should be low risk and I'd be happy to review a PR.