License the docs under GFDLv1.3+

Add a copy of the GNU Free Documentation License version 1.3, and
license the Jami manuals and documentation under the terms of the
GNU Free Documentation License, Version 1.3 or any later version
published by the Free Software Foundation; with no Invariant Sections,
no Front-Cover Texts, and no Back-Cover Texts.

Also, use the GNU All-Permissive License for the Makefile, conf.py,
and _templates/footer.html files since these files are rather small
and/or trivial.

GitLab: #3
Change-Id: If9f37afd66b9f415a9a4ac31ac037b9727fbb2e4