Add GitLab Duo runbook

Adds a runbook for GitLab Duo. This runbook should focus on issue triage for Duo features. The intended audience are product engineers and support engineers looking to resolve issues with degraded AI features on both gitlab.com and customer instances.

For Cloud Connector, I am adding a separate runbook here: !7765 (merged)

See also:

• gitlab-org/cloud-connector-team/team-tasks#177 (closed)
• gitlab-com/gl-infra/production#18329

Preview:

• triage.md