docs(cockpit): mark scaleway_cockpit resource as deprecated and add migration guidance

Author: jremy42

internal/services/cockpit/cockpit.go

@@ -90,6 +90,7 @@ func ResourceCockpit() *schema.Resource {
 				},
 			},
 		},
+		DeprecationMessage: "The scaleway_cockpit resource is deprecated and will be removed after January 1st, 2025. Use the new specialized resources instead: scaleway_cockpit_source, scaleway_cockpit_alert_manager, and scaleway_cockpit_grafana_user.",
 	}
 }