DOC: fix rpc reference in top-level index (#40077)
Summary:
Fixes gh-40046
PR gh-37419 refactored the content of `docs/source/rpc/index.rst` into `docs/source/rpc.rst` but did not link to the latter from `doc/source/index.rst` so the top-level RPC documentation is missing from https://pytorch.org/docs/master/.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40077
Differential Revision: D22068128
Pulled By: mrshenli
fbshipit-source-id: 394433f98f86509e0c9cb6d910a86fb8a2932683
