magma-1518-obstruction-lean: the announcement lives on the Zulip

The results of magma-1518-obstruction-lean went out on the Lean Zulip, and the repository briefly carried a draft of that announcement before it was removed the same morning: an announcement lives in the thread it was posted to, not as a copy in the repository that announces it. The same pass repointed the markdown notes at repository paths, so a reader followed a link into the repository rather than into a working copy.