Add runbook entries for all GCP alerts

Context

I'd like all alerts we receive in Slack to be actionable for every team member, regardless of their background with review-apps.

The alerts are being configured in https://gitlab.com/gitlab-org/quality/engineering-productivity-infrastructure/-/merge_requests/228, while this MR adds the RUNBOOK entries.

What does this MR do?

• Adds RUNBOOK entries for all alerts in GCP monitoring.
• Clean up some entries (see !42 (commits) for more details)

One more thing

I pushed to main by accident this morning, and I reverted those commits (see commit history on 2022-11-28: https://gitlab.com/gitlab-org/quality/engineering-productivity/team/-/commits/main).

I took the liberty of changing the repo settings a bit, so that it doesn't happen again:

Before

After