add preprocessor for RST docs, see #2461 and #2431

The idea is to filter the sources, replacing @<var>@ occurrences with
the mapping for <var> (which is currently hard-coded). @@ -> @. In order
to make this work, I had to move the doc sources one directory down
(into akka-docs/rst) so that the filtered result could be in a sibling
directory so that relative links (to _sphinx plugins or real code) would
continue to work.

While I was at it I also changed it so that WARNINGs and ERRORs are not
swallowed into the debug dump anymore but printed at [warn] level
(minimum).

One piece of fallout is that the (online) html build is now run after
the normal one, not in parallel.

akka-docs/rst/scala/code/docs/routing/RouterDocSpec.scala

@@ -0,0 +1,29 @@
+/**
+ * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+package docs.routing
+
+import RouterDocSpec.MyActor
+import akka.testkit.AkkaSpec
+import akka.routing.RoundRobinRouter
+import akka.actor.{ ActorRef, Props, Actor }
+
+object RouterDocSpec {
+  class MyActor extends Actor {
+    def receive = {
+      case _ ⇒
+    }
+  }
+}
+
+class RouterDocSpec extends AkkaSpec {
+
+  import RouterDocSpec._
+
+  //#dispatchers
+  val router: ActorRef = system.actorOf(Props[MyActor]
+    .withRouter(RoundRobinRouter(5, routerDispatcher = "router")) // “head” will run on "router" dispatcher
+    .withDispatcher("workers")) // MyActor workers will run on "workers" dispatcher
+  //#dispatchers
+
+}