Use GitLib exposed artifacts for third-party contributions

Pushing docs to docs.xmpp.rs requires access to the SSH private key, which is only granted to project members. Previously, that would make the docs build fail if non-members created an MR.

With this change, docs.xmpp.rs continues to be the default, but pipelines triggered by non-members will fall back to using GitLab exposed artifacts. The bot comment will point at the correct location either way.