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.
Also it avoids the failure mode where a user forgets to activate the
virtualenv, types the recommended "pip install" command, and installs
stuff directly to their system instead of safely confined inside the
virtualenv.
* use correct fixed-width-font markup
* fix hyperlinks to neighboring (github-side) .rst files
* refer to python-2.7.11 consistently (thanks to PRab for the catch)
