Merge branch 'russell/redirect_event_tracking' into 'master'

Add redirects to telemetry

See merge request gitlab-org/gitlab!27377

doc/development/event_tracking/backend.md

+---
+redirect_to: '../../telemetry/backend.md'
+---
+
+This document was moved to [another location](../../telemetry/backend.md).

doc/development/event_tracking/frontend.md

+---
+redirect_to: '../../telemetry/frontend.md'
+---
+
+This document was moved to [another location](../../telemetry/frontend.md).

doc/development/event_tracking/index.md

+---
+redirect_to: '../../telemetry/index.md'
+---
+
+This document was moved to [another location](../../telemetry/index.md).