Add a job to verify stage groups in runbooks

This adds a job to our scheduled pipeline to verify the stage-group mapping in the runbooks repo is up to date.

To do this, it triggers a pipeline in the runbooks repo: we went this way, because the mapping is generated in jsonnet. The tooling and fmt configuration lives in the runbooks repo, so copying over the data and image seemed more cumbersome.

This also makes our scheduled jobs triggerable from merge requests, because that makes it handy to verify changes.

Part of #663 (closed)