[docs] Reinstate r337730 - Add support for Markdown documentation in
Sphinx.

We think the bot is updated now, so trying this again. I'm landing it
(with permission) as Michael is at a con at the moment.

Actual patch largely by Michael Spencer.

Differential Revision: https://reviews.llvm.org/D44910

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338977 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed