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/scala/code/docs/camel/QuartzExample.scala

@@ -0,0 +1,30 @@
+package docs.camel
+
+object QuartzExample {
+  //#Quartz
+  import akka.actor.{ ActorSystem, Props }
+
+  import akka.camel.{ Consumer }
+
+  class MyQuartzActor extends Consumer {
+
+    def endpointUri = "quartz://example?cron=0/2+*+*+*+*+?"
+
+    def receive = {
+
+      case msg ⇒ println("==============> received %s " format msg)
+
+    } // end receive
+
+  } // end MyQuartzActor
+
+  object MyQuartzActor {
+
+    def main(str: Array[String]) {
+      val system = ActorSystem("my-quartz-system")
+      system.actorOf(Props[MyQuartzActor])
+    } // end main
+
+  } // end MyQuartzActor
+  //#Quartz
+}