Also removed the PIPENV_PYTHON suggestion, as this is likely to cause
confusion if added to e.g. .bashrc - it will always be set to the
global pyenv.
Fixes#4534
Removed ppa mentions from documentation, and re-organised the page a bit
to make it structurally easier to upderstand.
Both the documentation's front page and README.rst now actively refer to
the page so user can be directed to better installation methods. (I
don't think this is very useful TBT because people don't use manuals,
but we should still do it.)
Also fixed many Sphinx warnings during documentation builds.
Per Lundberg
kennethreitz