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/CustomRouteSample.java

@@ -0,0 +1,31 @@
+package docs.camel.sample.route;
+
+import akka.actor.*;
+import akka.camel.CamelExtension;
+
+public class CustomRouteSample {
+  public static void main(String[] args) {
+    try {
+      //#CustomRouteExample
+      // the below lines can be added to a Boot class, so that you can run the example from a MicroKernel
+      ActorSystem system = ActorSystem.create("some-system");
+      final ActorRef producer = system.actorOf(new Props(Producer1.class));
+      final ActorRef mediator = system.actorOf(new Props(new UntypedActorFactory() {
+        public Actor create() {
+          return new Transformer(producer);
+
+        }
+      }));
+      ActorRef consumer = system.actorOf(new Props(new UntypedActorFactory() {
+        public Actor create() {
+          return new Consumer3(mediator);
+
+        }
+      }));
+      CamelExtension.get(system).context().addRoutes(new CustomRouteBuilder());
+      //#CustomRouteExample
+    } catch (Exception e) {
+      e.printStackTrace();
+    }
+  }
+}