Add docs for peer-aggregated discovery

Author: richabanker

content/en/docs/concepts/architecture/mixed-version-proxy.md

@@ -108,3 +108,7 @@ serve the requested resource. This check happens using the internal
     to respond (for reasons such as network connectivity issues, or a data race between the request
     being received and a controller registering the peer's info into the control plane), then the handling
     API server responds with a 503 ("Service Unavailable") error.
+
+## Peer-aggregated discovery
+
+Peer-aggregated discovery provides a comprehensive cluster-wide view of all API resources, even during mixed-version upgrades when different API servers may serve different API groups.