Improve Clusters documentation

Avoid mentioning deprecated service

• Copy relevant info from the old k8s service to the cluster docs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16435

Getting the external IP address of a Kubernetes cluster

• Document how to get the external IP of the ingress deployed using the Kubernetes page: https://gitlab.com/gitlab-org/gitlab-ce/issues/42691
• Document how to use a static ip for GKE clusters: https://gitlab.com/gitlab-org/gitlab-ce/issues/42761