Documentation: submit-checklist: add clean builds for new Documentation

Add to Documentation/process/submit-checklist.rst that patch
submitters should run "make htmldocs" and verify that any
Documentation/ changes (patches) are clean (no new warnings/errors).

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/cf5bbdf5-03ff-0606-a6d4-ca196d90aee9@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>