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/sample/route/Consumer3.java

@@ -0,0 +1,27 @@
+package docs.camel.sample.route;
+
+//#CustomRouteExample
+import akka.actor.ActorRef;
+import akka.camel.CamelMessage;
+import akka.camel.javaapi.UntypedConsumerActor;
+
+public class Consumer3 extends UntypedConsumerActor{
+  private ActorRef transformer;
+
+  public Consumer3(ActorRef transformer){
+    this.transformer = transformer;
+  }
+
+  public String getEndpointUri() {
+    return "jetty:http://0.0.0.0:8877/camel/welcome";
+  }
+
+  public void onReceive(Object message) {
+    if (message instanceof CamelMessage) {
+      CamelMessage camelMessage = (CamelMessage) message;
+      transformer.forward(camelMessage.getBodyAs(String.class, getCamelContext()),getContext());
+    } else
+      unhandled(message);
+  }
+}
+//#CustomRouteExample