Replace the default Flutter starter README with comprehensive
documentation covering features, prerequisites, installation,
configuration, API endpoints, architecture, testing and build.
Upgrade flutter_web_auth_2 from ^5.0.3 to ^6.0.0-alpha.4 to resolve
the 'plugins that apply Kotlin Gradle Plugin' build warning.
This new version migrated to built-in Kotlin support as required by
Flutter SDK >=3.44.0.
Fixes #kotlin-gradle-plugin-warning