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/additional/code/osgi/Activator.scala

@@ -0,0 +1,25 @@
+package docs.osgi
+
+case object SomeMessage
+
+class SomeActor extends akka.actor.Actor {
+  def receive = { case SomeMessage ⇒ }
+}
+
+//#Activator
+import akka.actor.{ Props, ActorSystem }
+import org.osgi.framework.BundleContext
+import akka.osgi.ActorSystemActivator
+
+class Activator extends ActorSystemActivator {
+
+  def configure(context: BundleContext, system: ActorSystem) {
+    // optionally register the ActorSystem in the OSGi Service Registry
+    registerService(context, system)
+
+    val someActor = system.actorOf(Props[SomeActor], name = "someName")
+    someActor ! SomeMessage
+  }
+
+}
+//#Activator

akka-docs/rst/additional/code/osgi/blueprint.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0"
+           xmlns:akka="http://akka.io/xmlns/blueprint/v1.0.0">
+
+    <akka:actor-system name="BlueprintSystem" />
+
+    <akka:actor-system name="BlueprintSystemWithConfig">
+        <akka:config>
+            some.config {
+              key=value
+            }
+        </akka:config>
+    </akka:actor-system>
+</blueprint>