The staging installation of Jenkins by Retropikzel has proven itself
stable and convenient enough in our use that I suggest we add it as an
official service advertised on the Scheme.org front page next to Gitea.

As always, these services are opt-in for people who want to use them.
People who prefer to use something else for CI are welcome to do so.

The service is running on a Hetzner VPS rented by Retropikzel.

The PR is here: https://github.com/schemeorg/schemeorg/pull/48