Update incident management handbook to engage release managers for all S1 and S2 incidents

As part of the incident review for Incident Review: 500 error when sign-in with SA... (gitlab-com/gl-infra/production#17678 - closed) it was identified that we currently don't have guidance around engaging release manager for high severity incidents. severity1 and severity2 incidents do apply a blocks deployments tag but for incidents that may be escalated to higher severity, release managers may have additional mitigation options around rollbacks for IMOC.