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/quartz/QuartzSample.java

@@ -0,0 +1,12 @@
+package docs.camel.sample.quartz;
+//#QuartzExample
+import akka.actor.ActorSystem;
+import akka.actor.Props;
+
+public class QuartzSample {
+  public static void main(String[] args) {
+    ActorSystem system = ActorSystem.create("my-quartz-system");
+    system.actorOf(new Props(MyQuartzActor.class));
+  }
+}
+//#QuartzExample