docs: recommend ListenerSet over mergeGateways

The mergeGateways feature was introduced before the Gateway API
supported ListenerSet. ListenerSet is the Gateway API-native mechanism
for splitting listeners across resources: it removes the single-Gateway
64-listener limit and enables a delegated management model suitable for
high-scale, multi-tenant environments.

Add recommendation alerts to the deployment-mode docs pointing users to
ListenerSet, linking the upstream user guide and issue #1230.

Signed-off-by: Huabing (Robin) Zhao <zhaohuabing@gmail.com>

Author: zhaohuabing

site/content/en/latest/tasks/operations/deployment-mode.md

@@ -20,6 +20,16 @@ Please follow the example [Multi-tenancy](#multi-tenancy).
 
 ### Merged Gateways onto a single EnvoyProxy fleet
 
+{{% alert title="Recommendation" color="primary" %}}
+
+The `mergeGateways` feature was introduced before the Gateway API supported [ListenerSet][]. For splitting listeners
+across multiple resources, prefer [ListenerSet][]: it is the Gateway API-native mechanism, it removes the
+single-Gateway 64-listener limit, and it enables a delegated management model where a platform team owns the Gateway
+and application teams define ports, hostnames, and TLS certificates in their own ListenerSets, suitable for
+high-scale, multi-tenant environments. See the [ListenerSet user guide][] and [issue 1230][] for details.
+
+{{% /alert %}}
+
 By default, each Gateway has its own dedicated set of Envoy Proxy and its configurations.
 However, for some deployments, it may be more convenient to merge listeners across multiple Gateways and deploy a single Envoy Proxy fleet.
 
@@ -659,6 +669,12 @@ Handling connection for 8889
 
 ### Merged gateways deployment
 
+{{% alert title="Recommendation" color="primary" %}}
+
+For new deployments, consider using [ListenerSet][] instead of `mergeGateways`.
+
+{{% /alert %}}
+
 In this example, we will deploy GatewayClass
 
 ```shell
@@ -1083,3 +1099,5 @@ curl --header "Host: www.merged3.com" http://$GATEWAY_HOST:8082/example3
 [Gateway Namespace Mode]: ./gateway-namespace-mode
 [issue1231]: https://github.com/envoyproxy/gateway/issues/1231
 [issue2629]: https://github.com/envoyproxy/gateway/issues/2629
+[issue 1230]: https://github.com/envoyproxy/gateway/issues/1230
+[ListenerSet]: https://gateway-api.sigs.k8s.io/guides/user-guides/listener-set/