feat: Would like an explainer on releasing outside the stores

[eseidel]

We get regular requests on how to produce an apk to distribute outside the stores.

Another one this morning:

I distribute my app via Android APK only and getting user's to update their app is a struggle. My app is built with Flutter.

I wanted to confirm if Shorebird makes it possible for my user's app to always automatically update whenever I push a new build to production

A mention in https://docs.shorebird.dev/code_push/release about how to use --artifact apk or a dedicated guide on how to release for sideloading would help these users I think.