Give more detail on how to obtain fields when adding existing cluster

Improve https://docs.gitlab.com/ee/user/project/clusters/index.html#adding-an-existing-kubernetes-cluster

• Give some hints about how to get API URL, e.g. kubectl cluster-info
• Ditto for CA Certificate
• Explain how to create service account with correct permissions and get this token for RBAC enabled clusters

See https://docs.gitlab.com/ee/user/project/clusters/eks_and_gitlab/#connecting-the-eks-cluster for exact commands