Add metadata to Configure API pages

Claiming two API pages for the Configure team.

doc/api/group_clusters.md

+---
+stage: Configure
+group: Configure
+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
+---
+
 # Group clusters API
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/30213) in GitLab 12.1.

doc/api/project_clusters.md

+---
+stage: Configure
+group: Configure
+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
+---
+
 # Project clusters API
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/23922) in GitLab 11.7.