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/Consumer3.java

@@ -0,0 +1,31 @@
+package docs.camel;
+//#Consumer3
+import akka.actor.Status;
+import akka.camel.Ack;
+import akka.camel.CamelMessage;
+import akka.camel.javaapi.UntypedConsumerActor;
+
+public class Consumer3 extends UntypedConsumerActor{
+
+  @Override
+  public boolean autoAck() {
+    return false;
+  }
+
+  public String getEndpointUri() {
+    return "jms:queue:test";
+  }
+
+  public void onReceive(Object message) {
+    if (message instanceof CamelMessage) {
+      getSender().tell(Ack.getInstance(), getSelf());
+      // on success
+      // ..
+      Exception someException = new Exception("e1");
+      // on failure
+      getSender().tell(new Status.Failure(someException), getSelf());
+    } else
+      unhandled(message);
+  }
+}
+//#Consumer3