Add an S1 emergency runbook
Adding a sev-1 level incident remediation considerations for release managers.
This runbook is there to serve as a common team understanding on what to consider in situations such as S1 outage on GitLab.com.
Fixes https://gitlab.com/gitlab-com/gl-infra/delivery/issues/687
Dependent MR to add to handbook is in gitlab-com/www-gitlab-com!42702 (merged)
Edited by Marin Jankovski