Change divelogs.de export file suffix from .ZIP to .DLD

As Rainer finished up the export function on divelogs.de, he
used DLD as a suffix instead.

Suggested-by: Rainer Mohr <mail@divelogs.de>
Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Update user documentation with divelogs.de suffix change

divelogs.de use DLD as suffix, not ZIP

Suggested-by: Rainer Mohr <mail@divelogs.de>
Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>