Replace "Kubernetes cluster" where appropriate in docs

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/42939

doc/ci/README.md

@@ -71,9 +71,10 @@ learn how to leverage its potential even more.
   and status of each CI environment running on Kubernetes
 - [Trigger pipelines through the GitLab API](triggers/README.md)
 - [Trigger pipelines on a schedule](../user/project/pipelines/schedules.md)
-- [Deploy Boards](../user/project/deploy_boards.md) - Check the current health
 - [Kubernetes clusters](../user/project/clusters/index.md) - Integrate one or
   more Kubernetes clusters to your project
+- [Deploy Boards](../user/project/deploy_boards.md) - Check the current health
+  and status of each CI/CD environment running on Kubernetes
 
 ## GitLab CI/CD for Docker