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/java/code/docs/jrouting/FibonacciActor.java

@@ -0,0 +1,48 @@
+/**
+ * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+package docs.jrouting;
+
+import java.io.Serializable;
+
+import akka.actor.UntypedActor;
+
+//#fibonacciActor
+public class FibonacciActor extends UntypedActor {
+  public void onReceive(Object msg) {
+    if (msg instanceof FibonacciNumber) {
+      FibonacciNumber fibonacciNumber = (FibonacciNumber) msg;
+      getSender().tell(fibonacci(fibonacciNumber.getNbr()), getSelf());
+    } else {
+      unhandled(msg);
+    }
+  }
+
+  private int fibonacci(int n) {
+    return fib(n, 1, 0);
+  }
+
+  private int fib(int n, int b, int a) {
+    if (n == 0)
+      return a;
+    // recursion
+    return fib(n - 1, a + b, b);
+  }
+
+  public static class FibonacciNumber implements Serializable {
+    private static final long serialVersionUID = 1L;
+    private final int nbr;
+
+    public FibonacciNumber(int nbr) {
+      this.nbr = nbr;
+    }
+
+    public int getNbr() {
+      return nbr;
+    }
+
+  }
+}
+
+//#fibonacciActor
+