ci: test-docs-mr should not fail premerge pipelines
Sometimes premerge hardware testing shoots down innocent common code changes due to driver bugs, but it's a cost we pay in service of keeping regressions out. By contrast there's little compelling reason for documentation changes to shoot down merges, particularly merges that have nothing to do with docs but had the misfortune of touching a sphinx/doxygen tracked file. I'm not at all convinced that generating docs as CI artefacts is sane in the first place, but if we insist on doing so, it should at least not be in the critical path gating merges. The only jobs that should ever block merges are the ones where, should they fail, the result is sufficiently bad that the benefit of preventing regressions outweigh the inevitable noise. IMHO doxygen does not pass that bar.
https://gitlab.freedesktop.org/mesa/mesa/-/jobs/58300949 wasted a full merge. Warning, treated as error
adds insult to injury.
Short term action item would be demoting test-docs-mr to "post-merge or manual".