Add troubleshooting instructions to user manual

Let's cover at least the most common problem (modem manager interfering)
and also have instructions of how to dump debug log from the transfer.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Update Italian translation

(from Transifex so only with my SOB... I'm trying to figure out how to
give the translators correct 'git commit' credits :-) )

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Preparing for 3.1.1 and doc updates

This updates the versions to 3.1.1, does some minor doc cleanup for
consistency and adds the suggestion to submit the output of subsurface
--version in bug reports and the udev rules to blacklist dive computers in
Modem Manager (as suggested by Bearsch).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>