doc: re-add CodingStyle and SubmittingPatches

Those files got moved to Documentation/process, but as they're very
well known files, add pointers to their new locations.

PS.: I opted to not merge this patch with the previous one
in order to make the diff of the previous one more consistent,
as it will show only renames.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/CodingStyle

+This file has moved to process/coding-style.rst

Documentation/SubmittingPatches

+This file has moved to process/submitting-patches.rst