Document Cluster reference

[anmazzotti]

Even though the Cluster kind is technically part of provisioning.cattle.io API, it would be nice to have a full reference on the Elemental documentation.
A notable example can be found here: https://github.com/rancherfederal/rancher-cluster-templates/blob/main/charts/cluster-templates/values-elemental.yaml

Similarly we could add the full Cluster reference in the Elemental docs for ease of access.