Add feature flags details

Add details for previously added FFs

config/feature_flags/development/migrate_user_mentions.yml

 ---
 name: migrate_user_mentions
-introduced_by_url: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34378
 rollout_issue_url: 
-group: 
+group: group::project management
 type: development
 default_enabled: true

config/feature_flags/development/store_mentioned_users_to_db.yml

 ---
 name: store_mentioned_users_to_db
-introduced_by_url: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/19088
 rollout_issue_url: 
-group: 
+group: group::project management
 type: development
 default_enabled: true

ee/config/feature_flags/development/iteration_charts.yml

 ---
 name: iteration_charts
-introduced_by_url: 
-rollout_issue_url: 
-group: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41280
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/229046
+group: group::project management
 type: development
 default_enabled: false

ee/config/feature_flags/development/track_iteration_change_events.yml

 ---
 name: track_iteration_change_events
-introduced_by_url: 
-rollout_issue_url: 
-group: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/37620
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/229463
+group: group::project management
 type: development
 default_enabled: true