docs: process: submitting-patches: split canonical patch format section

docs: process: submitting-patches: split canonical patch format section

To make it easier to reference specific parts of the patch format,
let's add some headings for different parts.

Doing that, it becomes clear that backtraces in commit message is out of
place being after Reply-To Headers, so move it next to the commit
message body subsubsection.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.kernel.org/r/20241220-submitting-patches-imperative-v1-1-ee874c1859b3@pengutronix.de
Signed-off-by: Jonathan Corbet <corbet@lwn.net>