Improve and document how we publish brand flavours

[garethbowen]

When we tag a release of the app we update all brand flavours to their alpha channel which may be disruptive anyone using the alpha version. Adding a new flavour is done by forking, commenting out other brand flavours, and then tagging. This is complicated and dangerous.

Find a new way to release just desired brand and document how it's done.

[garethbowen]

Some ideas that have been running around in my head...

1. Use a parameterized GH action to take the flavour and release tag to build and allow manual build triggering so an app developer can do it without branching, tagging, or changing code in the repo. This could then also publish to the Play Store alpha channel (if desired).
2. Publishing the action as a workflow so it can be called from other repos. Then the flavour info can be stored in a separate repo with the rest of the app code and potentially in a private repo and trigger the action published in this repo. This would give the app developer full control over the app and not risk destabalising other apps.