[libcxx] Add new Sphinx documentation

Summary:
This patch adds Sphinx based documentation to libc++. The goal is to make it easier to write documentation for libc++ since writing new documentation in HTML is cumbersome. This patch rewrites the main page for libc++ along with the instructions for using, building and testing libc++. 

The built documentation can be found and reviewed here: http://efcs.ca/libcxx-docs

In order to build the sphinx documentation you need to specify the cmake options `-DLLVM_ENABLE_SPHINX=ON -DLIBCXX_INCLUDE_DOCS=ON`. This will add the makefile rule `docs-libcxx-html`.

Reviewers: chandlerc, mclow.lists, danalbert, jroelofs

Subscribers: silvas, cfe-commits

Differential Revision: http://reviews.llvm.org/D12129

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@245788 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/CMakeLists.txt b/docs/CMakeLists.txt
new file mode 100644
index 0000000..f63ee00
--- /dev/null
+++ b/docs/CMakeLists.txt
@@ -0,0 +1,9 @@
+
+if (LLVM_ENABLE_SPHINX)
+  if (SPHINX_FOUND)
+    include(AddSphinxTarget)
+    if (${SPHINX_OUTPUT_HTML})
+      add_sphinx_target(html libcxx)
+    endif()
+  endif()
+endif()
\ No newline at end of file