Instead of manually creating an empty branch and triggering a useless CI
build.

team#40516

See team#40512 for a replacement.

... and it's not for nothing! :)

The rationale is that (eventually? hopefully?) that documentation will
be irrelevant as all repos will have been migrated.

In general, we put migration docs in the *source* service, not the
target, because of that reason.

We keep the heading and a link to avoid breaking links.