[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

llvm-svn: 245788