Add reminder about /federate

Author: Bibas

pages/cockpit/reference-content/cockpit-supported-endpoints.mdx

@@ -72,6 +72,9 @@ Refer to the [official Mimir documentation](https://grafana.com/docs/mimir/lates
   - Path: `/federate`
 </Concept>
 
+<Message type="info">
+  The `/federate` endpoint aims to replicate the behavior of the classic Prometheus `/federate` endpoint, but it is not an official Mimir endpoint.
+</Message>
 
 <Message type="important">
   The `/federate` endpoint will not be billed during the beta phase. After the beta, the endpoint may incur additional costs.