Docs: Refactor adding and removing clusters doc

app/views/clusters/clusters/aws/_new.html.haml

doc/user/clusters/management_project.md

doc/user/group/clusters/index.md

doc/user/project/clusters/add_eks_clusters.md

doc/user/project/clusters/add_existing_cluster.md

doc/user/project/clusters/add_gke_clusters.md

doc/user/project/clusters/add_remove_clusters.md

doc/user/project/clusters/cluster_access.md

doc/user/project/clusters/index.md