Add info on the phabricator_import feature flag

The project has stalled, so I've linked the Epic for now, for when it
were to get picked up again, the issue for enabling the feature flag
could be added there.

config/feature_flags/development/phabricator_import.yml

 ---
 name: phabricator_import
-introduced_by_url: 
-rollout_issue_url: 
-group: 
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/13569
+rollout_issue_url: https://gitlab.com/groups/gitlab-org/-/epics/1197
+group: group::import
 type: development
 default_enabled: false