Update Cpp, rpc docs and Libraries section to match 1.5 (#38350)
Summary:
* Link cpp docs to the cpp landing page
* Link to rpc.rst landing page
* Update Libraries to match 1.5 (https://github.com/pytorch/pytorch/blob/release/1.5/docs/source/index.rst)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38350
Differential Revision: D21554435
Pulled By: jlin27
fbshipit-source-id: d1c9d5a86f84910225cbd0a57074ae95c8a9a450
