doc: Explain app versioning

[bryanoltman]

Topics to include (or at least include links to):

1. Shorebird uses the version reported by the compiled app to determine Release version
2. By default, this is the version in pubspec.yaml, BUT
3. It can be changed by the user (e.g., with fastlane)
4. How Info.plist and build.gradle consume version from pubspec (or not)

[bryanoltman]

Related to shorebirdtech/shorebird#2271