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/java/code/docs/camel/ProducerTestBase.java

@@ -0,0 +1,47 @@
+package docs.camel;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import scala.concurrent.Future;
+import akka.actor.ActorRef;
+import akka.actor.ActorSystem;
+import akka.actor.Props;
+import akka.camel.CamelMessage;
+import akka.pattern.Patterns;
+
+public class ProducerTestBase {
+  public void tellJmsProducer() {
+    //#TellProducer
+    ActorSystem system = ActorSystem.create("some-system");
+    Props props = new Props(Orders.class);
+    ActorRef producer = system.actorOf(props, "jmsproducer");
+    producer.tell("<order amount=\"100\" currency=\"PLN\" itemId=\"12345\"/>", null);
+    //#TellProducer
+    system.shutdown();
+  }
+
+  public void askProducer() {
+    //#AskProducer
+    ActorSystem system = ActorSystem.create("some-system");
+    Props props = new Props(FirstProducer.class);
+    ActorRef producer = system.actorOf(props,"myproducer");
+    Future<Object> future = Patterns.ask(producer, "some request", 1000);
+    //#AskProducer
+    system.stop(producer);
+    system.shutdown();
+  }
+
+  public void correlate(){
+    //#Correlate
+    ActorSystem system = ActorSystem.create("some-system");
+    Props props = new Props(Orders.class);
+    ActorRef producer = system.actorOf(props,"jmsproducer");
+    Map<String,Object> headers = new HashMap<String, Object>();
+    headers.put(CamelMessage.MessageExchangeId(),"123");
+    producer.tell(new CamelMessage("<order amount=\"100\" currency=\"PLN\" itemId=\"12345\"/>",headers), null);
+    //#Correlate
+    system.stop(producer);
+    system.shutdown();
+  }
+}