Add Sphinx documentation Factory and builders to zorg.

The *-sphinx-docs builders are being treated as experimental right now
so only myself and Sean Silva are notified. This is to avoid spamming
developers with documentation build failure notifications which might
discorage developers from writing documentation.

git-svn-id: https://llvm.org/svn/llvm-project/zorg/trunk@216063 91177308-0d34-0410-b5e6-96231b3b80d8