Add C++ Landing Page (#38450)
Summary:
* Add cpp_index.rst for landing page to match 1.5 (https://github.com/pytorch/pytorch/blob/release/1.5/docs/source/cpp_index.rst)
* Link to new cpp landing page was added to the docs table of contents in this PR: https://github.com/pytorch/pytorch/pull/38350
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38450
Differential Revision: D21580939
Pulled By: jlin27
fbshipit-source-id: 021c43f207a100d554266e4e16cb6752ca9c56a0
