Add metadata to prevent page being added to nav

This page needs to go away, but the merge request it's for is not
ready yet, so let's add metadata to prevent it from being automatically
added to the nav.

doc/user/project/merge_requests/reviews/data_usage.md

@@ -2,6 +2,7 @@
 stage: AI-powered
 group: AI Model Validation
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://handbook.gitlab.com/handbook/product/ux/technical-writing/#assignments
+ignore_in_report: true
 ---
 # Suggested Reviewers data usage