Merge branch 'russell/add-mmissing-metadata' into 'master'

Add missing metadata to documentation files

See merge request gitlab-org/gitlab!48213

doc/subscriptions/gitlab_com/index.md

 ---
-stage: none
-group: unassigned
+stage: fulfillment
+group: fulfillment
 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/#designated-technical-writers
 type: index, reference
 ---

doc/user/application_security/index.md

 ---
-stage: none
-group: unassigned
+stage: secure
+group: secure
 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/#designated-technical-writers
 type: reference, howto
 ---