Update and expand ai-assisted service runbooks

Problem

It was noticed in gitlab-com/gl-infra/production-engineering#27137 (closed) that our runbooks related to our ai-assisted service’s Apdex score fluctuations are sparse and have broken links.

Proposal

We should update and expand these runbooks to provide clearer, actionable steps for future incidents,

• Runbook Path: https://runbooks.gitlab.com/ai-assisted/#alerts
• Grafana dashboard: https://dashboards.gitlab.net/d/ai-assisted-main/ai-assisted3a-overview?from=2025-07-21T18:48:00.000Z&orgId=1&timezone=utc&to=2025-07-22T00:47:59.999Z&var-PROMETHEUS_DS=mimir-gitlab-gprd&var-environment=gprd&var-stage=main