doc: Use parallel builds

Using all your cores makes re-building the documentation a significantly
faster. It'd be even faster if sphinx would stop assuming every single
.rst file changes between builds, but casual googling didn't reveal a
fix so -EEFFORT. Might be a bug in Sphinx 1.8.3 which Fedora is shipping.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

doc/Makefile

@@ -2,7 +2,7 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
+SPHINXOPTS    = -j auto
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = _build