GitLab CI: build website
A second iteration could be to also run this job on MRs whose target branch is
master, in order to identify website build breakage before deployment.
A third iteration could be to run this job on all branches/MRs. In order to avoid wasting resources, we should have a mechanism to avoid building needlessly. For example, while building Tails images, we do this: https://gitlab.tails.boum.org/tails/tails/-/blob/stable/auto/scripts/website-cache.