build: Parallize the docs build.

There is no technical issue that prevents us from running this build in
parallel as far as I can tell. We can see if this will sufficiently
speed up the build for us to turn it on for PRs.

docs/Makefile

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