Add Secret Detection Validity Checks runbooks

Description

Create handbook runbooks for Secret Detection Validity Checks feature to enable self-service troubleshooting and reduce escalations.

Acceptance Criteria

• General FAQs runbook covers feature overview, partners, statuses, enabling/disabling
• Monitoring runbook documents dashboard metrics, alerts, and logs
• Troubleshooting runbook covers common issues: error rate, latency, rate limits, network errors
• Metrics documented against actual dashboard implementation (not fabricated)
• All runbooks follow GitLab runbook pattern with "When to use" section
• Runbooks use H3 headings, compact tables, minimal bold formatting
• Links to troubleshooting from monitoring alerts
• Escalation paths documented

Related

• Architecture: https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/secret_detection_validity_checks/
• Dashboard: https://dashboards.gitlab.net/d/secret-detection-sd-partner-token-verify/93b6ec2

Note: Used AI to write the doc.