Move the KAS installation docs under admin docs

From gitlab-com/www-gitlab-com!77218 (comment 531643981)

I think this would go into https://docs.gitlab.com/ee/user/clusters/agent/#install-the-kubernetes-agent-server - in a subsection, probably.

The section you linked is from the user facing docs, and only admins (not users) can install KAS. Maybe we should move the "Install the Kubernetes Agent Server" section to a new page under https://docs.gitlab.com/ee/administration/?

• Move the content over to administration: !57162 (merged)
• Update the navigation bar gitlab-docs!1652 (merged)