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/common/code/docs/circuitbreaker/CircuitBreakerDocSpec.scala

@@ -0,0 +1,43 @@
+/**
+ * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.circuitbreaker
+
+//#imports1
+import scala.concurrent.util.duration._ // small d is important here
+import akka.pattern.CircuitBreaker
+import akka.actor.Actor
+import scala.concurrent.Future
+import akka.event.Logging
+
+//#imports1
+
+class CircuitBreakerDocSpec {}
+
+//#circuit-breaker-initialization
+class DangerousActor extends Actor {
+
+  val log = Logging(context.system, this)
+  implicit val executionContext = context.dispatcher
+  val breaker =
+    new CircuitBreaker(context.system.scheduler, 5, 10.seconds, 1.minute)
+      .onOpen(notifyMeOnOpen)
+
+  def notifyMeOnOpen =
+    log.warning("My CircuitBreaker is now open, and will not close for one minute")
+  //#circuit-breaker-initialization
+
+  //#circuit-breaker-usage
+  def dangerousCall: String = "This really isn't that dangerous of a call after all"
+
+  def receive = {
+    case "is my middle name" ⇒
+      sender ! breaker.withCircuitBreaker(Future(dangerousCall))
+    case "block for me" ⇒
+      sender ! breaker.withSyncCircuitBreaker(dangerousCall)
+  }
+  //#circuit-breaker-usage
+
+}
+