Update API template to include metadata

As part of getting stage / group ownership metadata into all docs
pages, we need to update the template that generates
doc/api/graphql/reference/index.md

doc/api/graphql/reference/index.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
+---
+
 <!---
   This documentation is auto generated by a script.
 

lib/gitlab/graphql/docs/helper.rb

@@ -13,6 +13,12 @@ module Gitlab
 
         def auto_generated_comment
           <<-MD.strip_heredoc
+            ---
+            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
+            ---
+
             <!---
               This documentation is auto generated by a script.