update checker in the app that checks/installs from github directly

Hey,

I am looking for a stable version of the app where it's not needed to be downloaded from any app store (f-droid/google play store/aptoide/apkpure).

I have my android installed with lineageos but without the google services malware (I consider it to be a malware at this point), and the other app stores don't provide stable releases fast enough, after a release.

I noticed that you already put out release candidates on here, so why not provide an update checker in the app that goes to check github for new stable/beta releases? [FairEmail does that already](https://github.com/M66B/FairEmail#downloads): if you download it from the github release, it will keep checking for new stuff on there and completely skip and ignore any app stores.

This way you would have a way more streamlined process for app distribution.

If you accept Kotlin contributions, I might have some time to look into it as I do android here and there.