Document how to publish from other CI providers

[lucacasonato]

For example GitLab CI or Sourcehut. This requires #393.

[krlwlfrt]

I'd really like it if that feature would be implented, because my platform of choice is GitLab.

[ajhalili2006]

I may plan to write docs on that, although it may be complicated things if you want releases to be signed on sigstore.dev.