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/scala/code/docs/camel/PublishSubscribe.scala

@@ -1,47 +0,0 @@
-package docs.camel
-
-object PublishSubscribe {
-  {
-    //#PubSub
-    import akka.actor.{ Actor, ActorRef, ActorSystem, Props }
-    import akka.camel.{ Producer, CamelMessage, Consumer }
-
-    class Subscriber(name: String, uri: String) extends Actor with Consumer {
-      def endpointUri = uri
-
-      def receive = {
-        case msg: CamelMessage ⇒ println("%s received: %s" format (name, msg.body))
-      }
-    }
-
-    class Publisher(name: String, uri: String) extends Actor with Producer {
-
-      def endpointUri = uri
-
-      // one-way communication with JMS
-      override def oneway = true
-    }
-
-    class PublisherBridge(uri: String, publisher: ActorRef) extends Actor with Consumer {
-      def endpointUri = uri
-
-      def receive = {
-        case msg: CamelMessage ⇒ {
-          publisher ! msg.bodyAs[String]
-          sender ! ("message published")
-        }
-      }
-    }
-
-    // Add below to a Boot class
-    // Setup publish/subscribe example
-    val system = ActorSystem("some-system")
-    val jmsUri = "jms:topic:test"
-    val jmsSubscriber1 = system.actorOf(Props(new Subscriber("jms-subscriber-1", jmsUri)))
-    val jmsSubscriber2 = system.actorOf(Props(new Subscriber("jms-subscriber-2", jmsUri)))
-    val jmsPublisher = system.actorOf(Props(new Publisher("jms-publisher", jmsUri)))
-    val jmsPublisherBridge = system.actorOf(Props(new PublisherBridge("jetty:http://0.0.0.0:8877/camel/pub/jms", jmsPublisher)))
-    //#PubSub
-
-  }
-}