Add metadata to some Admin Area docs

This MR adds docs metadata to most of the docs in the
top-level `admin_area` directory.

doc/user/admin_area/abuse_reports.md

 ---
+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/#designated-technical-writers
 type: reference, howto
 ---
 

doc/user/admin_area/activating_deactivating_users.md

 ---
+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/#designated-technical-writers
 type: howto
 ---
 

doc/user/admin_area/blocking_unblocking_users.md

 ---
+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/#designated-technical-writers
 type: howto
 ---
 

doc/user/admin_area/credentials_inventory.md

+---
+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/#designated-technical-writers
+type: howto
+---
+
 # Credentials inventory **(ULTIMATE ONLY)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20912) in GitLab 12.6.

doc/user/admin_area/custom_project_templates.md

 ---
+stage: Plan
+group: Project Management
+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
 ---
 

doc/user/admin_area/diff_limits.md

 ---
+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/#designated-technical-writers
 type: reference
 ---
 

doc/user/admin_area/geo_nodes.md

 ---
+stage: Enablement
+group: Geo
+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: howto
 ---
 

doc/user/admin_area/labels.md

 ---
+stage: Plan
+group: Project Management
+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
 ---
 

doc/user/admin_area/license.md

 ---
+stage: Growth
+group: Conversion
+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: howto
 ---
 

doc/user/admin_area/merge_requests_approvals.md

 ---
+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/#designated-technical-writers
 type: reference, concepts
 ---