Improve documentation layout
Improve documentation layout

jungikim/OpenNMT,da03/OpenNMT,monsieurzhang/OpenNMT,jsenellart/OpenNMT,jsenellart/OpenNMT,monsieurzhang/OpenNMT,OpenNMT/OpenNMT,jungikim/OpenNMT,jsenellart-systran/OpenNMT,da03/OpenNMT,monsieurzhang/OpenNMT,da03/OpenNMT,jsenellart/OpenNMT,jungikim/OpenNMT,jsenellart-systran/OpenNMT,OpenNMT/OpenNMT,jsenellart-systran/OpenNMT,OpenNMT/OpenNMT