ghc-8.0.1: switch to sphinx for documentation

Fixes #20281

"Since GHC 8.0, the User’s Guide is authored in ReStructuredText (or ReST
or RST, for short) a rich but light-weight mark-up language aimed at
producing documentation. The Sphinx tool is used to produce the final
PDF and HTML documentation."
- http://ghc.readthedocs.io/en/8.0.1/editing-guide.html