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-remote/src/main/scala/akka/remote/RemoteActorRefProvider.scala

@@ -230,6 +230,8 @@ private[akka] class RemoteActorRefProvider(
 
     _log = Logging.withMarker(eventStream, getClass.getName)
 
+    showDirectUseWarningIfRequired()
+
     // this enables reception of remote requests
     transport.start()
 
@@ -265,6 +267,15 @@ private[akka] class RemoteActorRefProvider(
       remoteSettings.configureDispatcher(Props[RemoteDeploymentWatcher]()),
       "remote-deployment-watcher")
 
+  /** Can be overridden when using RemoteActorRefProvider as a superclass rather than directly */
+  protected def showDirectUseWarningIfRequired() = {
+    if (remoteSettings.WarnAboutDirectUse) {
+      log.warning(
+        "Using the 'remote' ActorRefProvider directly, which is a low-level layer. " +
+        "For most use cases, the 'cluster' abstraction on top of remoting is more suitable instead.")
+    }
+  }
+
   def actorOf(
       system: ActorSystemImpl,
       props: Props,