MyFirstContribution: recommend shallow threading of cover letters

MyFirstContribution: recommend shallow threading of cover letters

The "MyFirstContribution" document recommends the use of deep threading
of cover letters: every cover letter of subsequent iterations shall be
linked to the cover letter of the preceding version. The result of this
is that eventually, threads with many versions are getting nested so
deep that it becomes hard to follow.

Adapt the recommendation to instead propose shallow threading of cover
letters: instead of linking the cover letter to the previous cover
letter, the user is supposed to always link it to the first cover
letter. This still makes it easy to follow the iterations, but has the
benefit of nesting to a much shallower level.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>