docs: Tweak submitting-patches.rst formatting

The main goal here was to get the subsections to show in the TOC as they do
for all the other documents.  Also call out the DCO in the section title
since it's important.

Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>