Add metadata to files

Add Purchase group metadata to docs.

doc/subscriptions/index.md

 ---
-stage: none
-group: unassigned
+stage: Fulfillment
+group: Purchase
 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, reference
 ---

doc/subscriptions/self_managed/index.md

 ---
-stage: none
-group: unassigned
+stage: Fulfillment
+group: Purchase
 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, reference
 ---