Update the documentation to restrict the cluster-scoped Rollouts installation to user-defined namespace.

See https://docs.google.com/document/d/1ad-4aH7B1fJi7WEEKSzXNpHiz8sebfLCnpaZRhWqIoQ/edit and https://redhat-internal.slack.com/archives/C01RQH8KQ87/p1726093952356569 for details.

Doc updates required:

• We will need to ensure this is documented as a breaking change in 1.13/1.14 docs.
• The example RolloutManager CR on this page should be installed into openshift-gitops namespace. That is, .metadata.namespace should be 'openshift-gitops'
• We should find a spot in the Rollout docs to document the 'CLUSTER_SCOPED_ARGO_ROLLOUTS_NAMESPACES' env var, and how it can be used to install cluster-scoped Argo Rollouts to different namespaces.