Add information about mitigating bugs introduced by security merge request

For issue Create a process for addressing bugs introduced... (gitlab-com/gl-infra/delivery#19685 - closed)

This MR includes the documentation on the different options a stage team has when there is a bug introduced by a security merge request, as well as the runbook on how to revert a security merge request.

The template mentioned will be available once gitlab-org/gitlab!116615 (merged) is merged in.