Add metadata to pages

Add metadata to API docs.

doc/api/audit_events.md

 ---
-stage: none
-group: unassigned
+stage: Manage
+group: Compliance
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---
 

doc/api/groups.md

 ---
-stage: none
-group: unassigned
+stage: Manage
+group: Access
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---
 

doc/api/services.md

 ---
-stage: none
-group: unassigned
+stage: Create
+group: Ecosystem
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
 ---