doc-rst: add docutils config file

To stop the sphinx-build on severe errors and exit with an exit code (to
stop make) the halt_level must be set. The halt_level can't be set from
sphinx, it is a docutils configuration [1]. For this a docutils.conf was
added.

[1] http://docutils.sourceforge.net/docs/user/config.htmlSigned-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/docutils.conf

+# -*- coding: utf-8 mode: conf-colon -*-
+#
+# docutils configuration file
+# http://docutils.sourceforge.net/docs/user/config.html
+
+[general]
+halt_level: severe
\ No newline at end of file