Remove references to Metrics Dashboard Annotations from docs

What does this MR do and why?

Changes: Removes the metrics_dashboard_annotations API from the docs. It should have been removed many months ago. Nav removal MR.

Context: This MR cleans up a portion of dead code related to the removal of devopsmonitor Category:Metrics features. The Metrics Dashboard & Prometheus Metrics/Alerts (used for monitoring apps deploying using gitlab) were deprecated in 14.7 and removed from the product in 16.0.

• grouprespond related issue: Metrics: Delete models & database tables/columns (#397142 - closed)
• Monitor:Metrics removal epic: Monitor: Metrics Deprecation and Removal (&10107 - closed)
• Reference MR with all related removals: Draft: Remove all remaining metrics dashboard-r... (!160325 - closed)
• Metrics removal is blocking cells: Drop old unused tables for prometheus alerts (#461838 - closed)

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