docs: Initial Sphinx setup for Clang.

I don't think this will be visible just yet on <clang.llvm.org/docs/>
since I don't think that the necessary server-side setup has taken
place.

Don't shoot me over the theme. I don't want to duplicate LLVM's theme
into the clang repo at the moment, so I just used one of Sphinx's
default themes.

llvm-svn: 170042