Document the task expiry setting

https://redmine.teklia.com/issues/7332

Requires backend#1756 (closed)

Please document the ponos.task_expiry setting in the configuration page.

changed milestone to %Arkindex 1.6.1

added P2 Quick Win labels

assigned to @vrigal

mentioned in merge request !131 (merged)

closed with merge request !131 (merged)