GitLab CI: build website
A first iteration could be to merely migrate what we have on Jenkins (job definition, that simply runs ./build-website) to GitLab CI: build the website on the master branch.
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.
cc @zen
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