Merge branch 'docs/ssot_group_clusters' into 'master'

Edited "Group cluster" for SSOT guidelines

See merge request gitlab-org/gitlab-ce!29176

doc/user/group/clusters/index.md

+---
+type: reference
+---
+
 # Group-level Kubernetes clusters
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/34758) in GitLab 11.6.