Update libcxx.llvm.org documentation by linking to new docs.

Summary:
Currently much of the libcxx website is duplicated between the old www/ documentation and newer Sphinx docs. This patch changes the main libc++ webpage so that it links to the new documentation where possible. This means removing numerous sections from the landing page.

@mclow.lists What do you think?

Reviewers: mclow.lists

Subscribers: cfe-commits, mclow.lists

Differential Revision: http://reviews.llvm.org/D19250

llvm-svn: 271469