Update distributed autograd design doc with appropriate links. (#29927)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29927
With the docs page now up, we can update the links in the design doc
to point to the docs page.
ghstack-source-id: 94055423
Test Plan: waitforbuildbot
Differential Revision: D18541878
fbshipit-source-id: f44702d9a8296ccc0a5d58d56c3b6dc8a822b520
