Add systemd timer instructions for mirrors

https://wiki.almalinux.org/Mirrors.html

Due to some confusion with the crontab line using a `%`, which behaves differently outside of a cron, we need to add an alternative way to do this via a systemd timer.

Example instances of confusion:

* https://github.com/AlmaLinux/wiki/issues/275
* https://chat.almalinux.org/almalinux/pl/xo1snp586pyjuju7hi6a9894ph
* https://chat.almalinux.org/almalinux/pl/yzpaztd377rtt89pz4xx84ybdh