This uses Read-The-Docs (sphinx/docutils) references exclusively, but adds a
README.md for GitHub viewers to remind them that the links there won't
work (closes ticket:2835).
It also fixes all the dangling references and other Sphinx warnings.
The "Preparation" section of docs/magic-folder-howto.rst was removed, since
this feature has since been merged to trunk.
Add ".. -*- coding: utf-8-with-signature -*-" to the first line of each .rst
file. This tells emacs to treat the file contents as utf-8, and also to prepend
a so-called utf-8 "bom" marker at the beginning of the file. This patch also
prepends those markers to each of those files.
