Update docs CI for v1.3.0 (#27638)
This PR updates the docs CI. After this is merged, we open a PR from
1.3.0 -> master. That open PR will build docs on this branch and push
them to pytorch.github.io:site-v1.3.0. This is done in dry_run mode
so the pushing won't actually happen; I will follow up with a
subsequent change to drop dry_run mode after verifying that everything
builds correctly.
