Split up 'Networking' docs in 'Networking' and 'Clustering' (#24142)

We also probably want to add some more wording explaining you probably don't
want to use remoting directly, but either use a 'proper' protocol like
akka-http to loosely couple or akka-cluster for more coupled systems, but that
could be another PR

Move 'networking' below 'streams' as you don't typically have to deal with it directly

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

@@ -0,0 +1,17 @@
+# Clustering
+
+@@toc { depth=2 }
+
+@@@ index
+
+* [common/cluster](common/cluster.md)
+* [cluster-usage](cluster-usage.md)
+* [cluster-singleton](cluster-singleton.md)
+* [distributed-pub-sub](distributed-pub-sub.md)
+* [cluster-client](cluster-client.md)
+* [cluster-sharding](cluster-sharding.md)
+* [cluster-metrics](cluster-metrics.md)
+* [distributed-data](distributed-data.md)
+* [cluster-dc](cluster-dc.md)
+
+@@@

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

@@ -4,15 +4,6 @@
 
 @@@ index
 
-* [common/cluster](common/cluster.md)
-* [cluster-usage](cluster-usage.md)
-* [cluster-singleton](cluster-singleton.md)
-* [distributed-pub-sub](distributed-pub-sub.md)
-* [cluster-client](cluster-client.md)
-* [cluster-sharding](cluster-sharding.md)
-* [cluster-metrics](cluster-metrics.md)
-* [distributed-data](distributed-data.md)
-* [cluster-dc](cluster-dc.md)
 * [remoting](remoting.md)
 * [remoting-artery](remoting-artery.md)
 * [serialization](serialization.md)
@@ -22,4 +13,4 @@
 * [camel](camel.md)
 * [multi-jvm-testing](multi-jvm-testing.md)
 
-@@@
+@@@

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

@@ -8,8 +8,9 @@
 * [guide/index](guide/index.md)
 * [general/index](general/index.md)
 * [index-actors](index-actors.md)
-* [index-network](index-network.md)
+* [index-cluster](index-cluster.md)
 * [stream/index](stream/index.md)
+* [index-network](index-network.md)
 * [index-futures](index-futures.md)
 * [index-utilities](index-utilities.md)
 * [common/other-modules](common/other-modules.md)