Documentation/sphinx: change pdflatex interaction mode to batchmode

Radically reduce the noise on stdout. The full build logs will still be
available under Documentatio/output/latex/*.log.

Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Tested-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>