main/mandoc: periodically run makewhatis via a cron

Closed Sören Tempel requested to merge nmeum/aports:mandoc-cron into master

The trigger takes a quite a while to run at times. Furthermore, I would argue not vital for the apropos database to be always up to date, therefore it's fine to run it via a cron instead. This should be especially true since f0fb5efa.

Prior art:

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