Refactor  Cluster docs

Closes #39952

See merge request gitlab-org/gitlab-ce!15418

(cherry picked from commit d41e66cb)

644a2941 Add info about the installed apps in the Cluster docs
b7a01e08 Add info on creating your own k8s cluster from the cluster page
ee7ecc81 Add Ingress description from official docs
3b9773fc Change to Google Kubernetes Cluster and add internal links
bacf671a Move installation of apps higher