Create a playbook for how to deal with non-succeeding PRs

We should have a playbook especially for how to deal with PR failures. Some of them like #2941 can break every workflow by failing the tests.

Things that should be included in such a playbook would be especially emergency procedures
- Short term mitigation: What tests are optional and can safely be removed if no one with the necessary knowledge is available to fix the issue
- What needs to be done, like sync the branch to the newest commit if PRs fail but a fix in the repository has been submitted already