Create runbook for container registry migration phase 2
Context
Followup from gitlab-com/gl-infra/readiness!109 (comment 901344818).
Related to gitlab-com/gl-infra/readiness#30 (closed).
Proposal
Create a runbook that lists:
- The feature flags and settings at our disposal to manage the rollout;
- Links for logs and relevant dashboards;
- Useful commands/queries to check various things on Rails and registry.