monitor some key APKs

Continuing from mirror-monitor#3 (comment 1589130746):

A nice start would be to monitor some key APKs, like org.fdroid.fdroid

Happy to do that, but AFAICT that means we'd have to manually update the URL(s) being monitored when the current version changes (though I could probably set up another scheduled job to alert us when that's needed like for mirror list changes in the index).