Move 'Remoting' under 'Clustering' in the docs (#26774)

* Move 'Remoting' under 'Clustering' in the docs #26491

* As it is typically not used directly, but as an implementation detail of
Clustering.

* Warn when remoting is used directly #26491

akka-docs/src/main/paradox/index-cluster.md

@@ -16,5 +16,8 @@
 * [cluster-dc](cluster-dc.md)
 * [multi-jvm-testing](multi-jvm-testing.md)
 * [multi-node-testing](multi-node-testing.md)
+* [remoting](remoting.md)
+* [remoting-artery](remoting-artery.md)
+* [serialization](serialization.md)
 
 @@@

akka-docs/src/main/paradox/index-network.md

@@ -4,9 +4,6 @@
 
 @@@ index
 
-* [remoting](remoting.md)
-* [remoting-artery](remoting-artery.md)
-* [serialization](serialization.md)
 * [io](io.md)
 * [io-tcp](io-tcp.md)
 * [io-udp](io-udp.md)

akka-docs/src/main/paradox/remoting-artery.md

@@ -1,5 +1,18 @@
 # Remoting (codename Artery)
 
+@@@ note
+
+Remoting is the mechanism by which Actors on different nodes talk to each
+other internally.
+
+When building an Akka application, you would usually not use the Remoting concepts
+directly, but instead use the more high-level
+@ref[Akka Cluster](index-cluster.md) utilities or technology-agnostic protocols
+such as [HTTP](https://doc.akka.io/docs/akka-http/current/),
+[gRPC](https://developer.lightbend.com/docs/akka-grpc/current/) etc.
+
+@@@
+
 ## Dependency
 
 To use Remoting (codename Artery), you must add the following dependency in your project:

akka-docs/src/main/paradox/remoting.md

@@ -1,5 +1,19 @@
 # Remoting
 
+@@@ note
+
+Remoting is the mechanism by which Actors on different nodes talk to each
+other internally.
+
+When building an Akka application, you would usually not use the Remoting concepts
+directly, but instead use the more high-level
+@ref[Akka Cluster](index-cluster.md) utilities or technology-agnostic protocols
+such as [HTTP](https://doc.akka.io/docs/akka-http/current/),
+[gRPC](https://developer.lightbend.com/docs/akka-grpc/current/) etc.
+
+
+@@@
+
 ## Dependency
 
 To use Akka Remoting, you must add the following dependency in your project: