Do not deploy to pages if the docs:build job fails

This actually requires to adjust allow_failure such that failure is not allowed — this makes most sense, because we do not want to be able to merge MRs that corrupt the docs, so we shouldn't have allow_failure enabled.

This MR was prompted by a failing docs job which led to docs.utopia-project.org showing a 404. The underlying reason is a regression in xarray dependency , which is already fixed but not yet deployed. Either way: We should take care to not be susceptible to these kind of errors destroying our docs page.