Change Sphinx configurations a bit, this is a first step to provide i18n for the manual
Provide translation templates for the manual. Install ``sphinx-intl``, see http://sphinx-doc.org/intl.html. Use ``sphinx-intl update -p ../build/locale/``to update the templates. These templates were manually uploaded to the Transifex translation service for now, just to get things started. See https://www.transifex.com/projects/p/mixxxdj-manual/ . Next step is to setup Transifex to automatically pick up changes to the translation templates @mixxxdj/manual, like we do for @mixxxdj/mixxx