Fixed Sphinx build for Client Manual

Copied in _shared_assets to get themes, Makefile, renamed conf.py.in to conf.py, edited conf.py to point to themes in _shared_assets. make html and make pdf both work. The PDF does not pick up the version number, but rather prints Release @HEADER@ on all the page headers.

This looks like a lot of changes, but it's mainly just copied files. The only file in this PR that was edited is conf.py. Everything else was copied from owncloud/documentation.

Please review this @danimo and anyone who speaks Sphinx.