Add Plan:Certify metadata

doc/administration/reply_by_email.md

+---
+stage: Plan
+group: Certify
+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
+---
+
 # Reply by email
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/1173) in GitLab 8.0.