This way we can have a task to immediately destroy a repo, and it
clarifies what the task does.

We want an "immediate" job because we do want to trash some repos and
there might be some checks to do then.

back in 9d9aed39 (use <a id=""> instead of name, as name is not HTML5,
2023-06-01) we switched everything to id= but overlooked that id=
actually gets redacted by GitLab's sanitizer. With this, we should be
able to get the anchors through the filter even if it generates
non-conformant HTML5.

The "how to" bit is redundant. We keep an anchor for backwards
compatibility.

All the howtos were under the "Commit hooks" docs, weird.

See: team#41180

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.