#22903 Fix code snippeds

* move scala and java sources out of paradox sources
* replace relative paths with $code$ and $akka$
* fix broken includes

akka-docs/src/main/scala/docs/stream/cookbook/RecipeKeepAlive.scala

@@ -0,0 +1,25 @@
+package docs.stream.cookbook
+
+import akka.NotUsed
+import akka.stream.ClosedShape
+import akka.stream.scaladsl._
+import akka.util.ByteString
+
+class RecipeKeepAlive extends RecipeSpec {
+
+  "Recipe for injecting keepalive messages" must {
+
+    "work" in {
+      val keepaliveMessage = ByteString(11)
+
+      //#inject-keepalive
+      import scala.concurrent.duration._
+      val injectKeepAlive: Flow[ByteString, ByteString, NotUsed] =
+        Flow[ByteString].keepAlive(1.second, () => keepaliveMessage)
+      //#inject-keepalive
+
+      // No need to test, this is a built-in stage with proper tests
+    }
+  }
+
+}