main/mandoc: periodically run makewhatis via a cron

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: https://lists.alpinelinux.org/~alpine/aports/%3C1439237974-14854-1-git-send-email-soeren%2Bgit%40soeren-tempel.net%3E