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/general/code/docs/config/ConfigDoc.java

@@ -1,33 +0,0 @@
-/**
- * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
- */
-package docs.config;
-
-import akka.actor.ActorSystem;
-import com.typesafe.config.*;
-
-public class ConfigDoc {
-  public ActorSystem createConfiguredSystem() {
-    //#java-custom-config
-    // make a Config with just your special setting
-    Config myConfig =
-      ConfigFactory.parseString("something=somethingElse");
-    // load the normal config stack (system props,
-    // then application.conf, then reference.conf)
-    Config regularConfig =
-      ConfigFactory.load();
-    // override regular stack with myConfig
-    Config combined =
-      myConfig.withFallback(regularConfig);
-    // put the result in between the overrides
-    // (system props) and defaults again
-    Config complete =
-      ConfigFactory.load(combined);
-    // create ActorSystem
-    ActorSystem system =
-      ActorSystem.create("myname", complete);
-    //#java-custom-config
-    return system;
-  }
-}
-