Make a simple build-my-own-release flow

[remstos]

Goal is:

• I currently use the last Beta we ever built, I should be able to replace it with a custom build I make with Xcode from the OSS project.

We won’t have a full release flow soon.

But we should have a simple flow + explanation to build a correctly working local beta version.

[McNight]

We definitely should take advantage of Github Actions to manage this (and more)