Improve DB incident management

Database incidents typically have high severity. It is very important to handle those incidents as efficient as possible. Some possible improvements:

• make it easy to escalate and reach out for support/help
  1. better documentation on how to page OnGres (e.g. on the Incident Management handbook)
  2. add paging instructions to #ongres-gitlab slack channel title
  3. ways to reach out to database team on weekends?

corrective action for production#2885 (closed)