Document new options added for cli-utils GitOps

Document new options added in gitlab-org/cluster-integration/gitlab-agent!376 (merged), gitlab-org/cluster-integration/gitlab-agent!386 (merged).
Document how to adopt the agent if the cluster is already set up with or without an inventory template. !64155 (merged)

Edited Jul 08, 2021 by Mikhail Mazurskiy