Add front matter for 3 new pages

These pages need to be tracked by the technical writing team.
Add front matter to do so.

doc/administration/integration/kroki.md

+---
+stage: Create
+group: Ecosystem
+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
+---
+
 # Kroki diagrams **(FREE SELF)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/241744) in GitLab 13.7.

doc/api/graphql/removed_items.md

+---
+stage: Plan
+group: Product Planning
+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
+---
+
 # GraphQL API removed items
 
 GraphQL is a versionless API, unlike the REST API.

doc/development/integrations/codesandbox.md

+---
+stage: none
+group: Development
+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
+---
+
 # Set up local Codesandbox development environment
 
 This guide walks through setting up a local [Codesandbox repository](https://github.com/codesandbox/codesandbox-client) and integrating it with a local GitLab instance. Codesandbox