Merge branch 'docs-system-hooks-metadata' into 'master'

Add metadata to System hooks page

See merge request gitlab-org/gitlab!71821

doc/system_hooks/system_hooks.md

 ---
-stage: none
-group: unassigned
+stage: Ecosystem
+group: Integrations
 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
 ---