Add stage / group metadata to pages

These pages belong to Source Code, and now the metadata reflect it.

doc/api/templates/dockerfiles.md

 ---
-stage: none
-group: unassigned
+stage: Create
+group: Source Code
 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
 type: reference
 ---

doc/user/admin_area/settings/index.md

 ---
-stage: none
-group: unassigned
+stage: Create
+group: Source Code
 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
 type: index
 ---