Fix broken documentation links in alert_management.go

The documentation links in the AlertManagementService are currently pointing to non-existent anchors.

This is caused by recent changes in the GitLab API documentation structure where anchors were renamed to be more descriptive (e.g., adding "a" or "all" to the slug).

Affected Links:

delete-metric-image -> delete-a-metric-image

list-metric-images -> list-all-metric-images

update-metric-image -> update-a-metric-image