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/howto.rst

@@ -0,0 +1,36 @@
+
+.. _howto-java:
+
+######################
+HowTo: Common Patterns
+######################
+
+This section lists common actor patterns which have been found to be useful,
+elegant or instructive. Anything is welcome, example topics being message
+routing strategies, supervision patterns, restart handling, etc. As a special
+bonus, additions to this section are marked with the contributor’s name, and it
+would be nice if every Akka user who finds a recurring pattern in his or her
+code could share it for the profit of all. Where applicable it might also make
+sense to add to the ``akka.pattern`` package for creating an `OTP-like library
+<http://www.erlang.org/doc/man_index.html>`_.
+
+You might find some of the patterns described in the Scala chapter of 
+:ref:`howto-scala` useful even though the example code is written in Scala.
+
+Template Pattern
+================
+
+*Contributed by: N. N.*
+
+This is an especially nice pattern, since it does even come with some empty example code:
+
+.. includecode:: code/docs/pattern/JavaTemplate.java
+   :include: all-of-it
+   :exclude: uninteresting-stuff
+
+.. note::
+
+   Spread the word: this is the easiest way to get famous!
+
+Please keep this pattern at the end of this file.
+