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/pattern/ScalaTemplate.scala

@@ -0,0 +1,16 @@
+/**
+ *  Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.pattern
+
+// this part will not appear in the docs
+
+//#all-of-it
+class ScalaTemplate {
+  println("Hello, Template!")
+  //#uninteresting-stuff
+  // don’t show this plumbimg
+  //#uninteresting-stuff
+}
+//#all-of-it