Add guide for annotations in cedar terraform provider

[ckluy31]

Tested that output was correct using terraform plan:

Changes to Outputs:
  + policy_text = <<-EOT
        @advice("Allow admins to read public resources unless owned by Alice")
        permit (
                principal in Group::"admins",
                action == Action::"Read",
                resource
        )
        when {
                resource.is_public
        }
        unless {
                resource.owner == User::"alice"
        };
    EOT