docs: Fix broken GitLab docs anchors for alert_management API
What does this MR do?
Fixes broken anchor links to the GitLab Wikis API documentation in alert_management.go.
Several links were pointing to outdated or incorrect section anchors, which caused navigation to land at the top of the page instead of the intended API section.
What was changed?
Updated the following documentation links to match the current GitLab docs anchors:
- ListMetricImages https://docs.gitlab.com/api/alert_management_alerts/#list-all-metric-images
- UpdateMetricImage https://docs.gitlab.com/api/alert_management_alerts/#update-a-metric-image
- DeleteMetricImage https://docs.gitlab.com/api/alert_management_alerts/#delete-a-metric-image