[torch/elastic] Update the rendezvous docs (#58160)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58160
This PR updates the Torch Distributed Elastic documentation with references to the new `c10d` backend.
ghstack-source-id: 128783809
Test Plan: Visually verified the correct
Reviewed By: tierex
Differential Revision: D28384996
fbshipit-source-id: a40b0c37989ce67963322565368403e2be5d2592
