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/camel/HttpExample.scala

@@ -0,0 +1,47 @@
+package docs.camel
+
+object HttpExample {
+
+  {
+    //#HttpExample
+    import org.apache.camel.Exchange
+    import akka.actor.{ Actor, ActorRef, Props, ActorSystem }
+    import akka.camel.{ Producer, CamelMessage, Consumer }
+    import akka.actor.Status.Failure
+
+    class HttpConsumer(producer: ActorRef) extends Consumer {
+      def endpointUri = "jetty:http://0.0.0.0:8875/"
+
+      def receive = {
+        case msg ⇒ producer forward msg
+      }
+    }
+
+    class HttpProducer(transformer: ActorRef) extends Actor with Producer {
+      def endpointUri = "jetty://http://akka.io/?bridgeEndpoint=true"
+
+      override def transformOutgoingMessage(msg: Any) = msg match {
+        case msg: CamelMessage ⇒ msg.addHeaders(msg.headers(Set(Exchange.HTTP_PATH)))
+      }
+
+      override def routeResponse(msg: Any) { transformer forward msg }
+    }
+
+    class HttpTransformer extends Actor {
+      def receive = {
+        case msg: CamelMessage ⇒ sender ! (msg.mapBody { body: Array[Byte] ⇒ new String(body).replaceAll("Akka ", "AKKA ") })
+        case msg: Failure      ⇒ sender ! msg
+      }
+    }
+
+    // Create the actors. this can be done in a Boot class so you can
+    // run the example in the MicroKernel. just add the below three lines to your boot class.
+    val system = ActorSystem("some-system")
+    val httpTransformer = system.actorOf(Props[HttpTransformer])
+    val httpProducer = system.actorOf(Props(new HttpProducer(httpTransformer)))
+    val httpConsumer = system.actorOf(Props(new HttpConsumer(httpProducer)))
+    //#HttpExample
+
+  }
+
+}