Typed default blocking dispatcher + docs (#25359)

Closes #24893

akka-docs/src/main/paradox/typed/dispatchers.md

@@ -0,0 +1,44 @@
+# Dispatchers
+
+## Dependency
+
+Dispatchers are part of core akka, which means that they are part of the akka-actor-typed dependency:
+
+@@dependency[sbt,Maven,Gradle] {
+  group="com.typesafe.akka"
+  artifact="akka-actor-typed_$scala.binary_version$"
+  version="$akka.version$"
+}
+
+## Introduction 
+
+An Akka `MessageDispatcher` is what makes Akka Actors "tick", it is the engine of the machine so to speak.
+All `MessageDispatcher` implementations are also an @scala[`ExecutionContext`]@java[`Executor`], which means that they can be used
+to execute arbitrary code, for instance @ref:[Futures](../futures.md).
+
+## Selecting a dispatcher
+
+A default dispatcher is used for all actors that are spawned without specifying a custom dispatcher.
+This is suitable for all actors that don't block. Blocking in actors needs to be carefully managed, more
+details @ref:[here](../dispatchers.md#blocking-needs-careful-management).
+
+To select a dispatcher use `DispatcherSelector` to create a `Props` instance for spawning your actor:
+
+Scala
+:  @@snip [DispatcherDocSpec.scala]($akka$/akka-actor-typed-tests/src/test/scala/docs/akka/typed/DispatchersDocSpec.scala) { #spawn-dispatcher }
+
+Java
+:  @@snip [DispatcherDocTest.java]($akka$/akka-actor-typed-tests/src/test/java/jdocs/akka/typed/DispatchersDocTest.java) { #spawn-dispatcher }
+
+`DispatcherSelector` has two convenience methods to look up the default dispatcher and a dispatcher you can use to 
+execute actors that block e.g. a legacy database API that does not support @scala[`Future`]@java[`CompletionStage`]s.
+
+The final example shows how to load a custom dispatcher from configuration and replies on this being in your application.conf:
+
+Scala
+:  @@snip [DispatcherDocSpec.scala]($akka$/akka-actor-typed-tests/src/test/scala/docs/akka/typed/DispatchersDocSpec.scala) { #config }
+
+Java
+:  @@snip [DispatcherDocSpec.scala]($akka$/akka-actor-typed-tests/src/test/scala/docs/akka/typed/DispatchersDocSpec.scala) { #config }
+
+For full details on how to configure custom dispatchers see the @ref:[untyped docs](../dispatchers.md#types-of-dispatchers).

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

@@ -5,6 +5,7 @@
 @@@ index
 
 * [actors](actors.md)
+* [dispatchers](dispatchers.md)
 * [coexisting](coexisting.md)
 * [actor-lifecycle](actor-lifecycle.md)
 * [interaction patterns](interaction-patterns.md)