Adds first version of DCR runbook

This MR addresses gitlab-org/gitlab#552385 (closed).

This change creates a first runbook version with the addition of a troubleshooting guide for Duo Code Review.

The new documentation provides support and engineering teams with essential information including how to monitor the system through various dashboards, common problems users might encounter and their solutions, technical requirements like licensing and permissions, and step-by-step instructions for investigating issues when they occur.

It also includes contact information for escalating complex problems to the Code Creation team and links to user documentation for self-service support.