Add temporary Makefile.sphinx build file to get libcxx.llvm.org/docs going

llvm-svn: 246909
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 5fb53fce5f5bd67adcc971fcb9a3ddcd5ee9b7b5
1 file changed