Mark page as part of Configure docset

This page was added by a recent set of Configure merge requests, so
it clearly belongs to the Configure stage.

doc/api/instance_clusters.md

 ---
-stage: none
-group: unassigned
+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
 ---