Documentation/maintainer: rehome sign-off process

The repeated sign-offs necessary when a subsystem maintainer modifies an
incoming patch has been moved from submitting-patches.rst to
Documentation/maintainer, since the affairs of a subsystem maintainer
are not especially relevant to someone reading a guide for how to submit
their first patch.

Signed-off-by: Drew DeVault <sir@cmpwn.com>
Link: https://lore.kernel.org/r/20200903160545.83185-4-sir@cmpwn.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>