feat/new arch v4 docs

[TomWq]

Summary

This PR upgrades react-native-splash-screen to the v4 line with React Native New Architecture support (TurboModule + Codegen), and modernizes the example app and documentation.

What changed

New Architecture migration

• Add codegen setup and module spec for SplashScreen
• Export TurboModule-backed API in JS, with compatibility handling
• Android:
  • migrate package/module implementation for New Architecture
  • wire codegen/autolinking (including libraryName and JNI CMake setup)
• iOS:
  • migrate native module to New Architecture-compatible implementation
  • update podspec integration for module dependencies

Example app updates

• Upgrade example app to modern RN template (0.84)
• Keep splash behavior working on both Android and iOS
• Update launch screen assets/layout usage for image-based splash

Documentation refresh

• Rewrite main docs for v4 (RN >= 0.84, New Architecture only)
• Add explicit notice that v4 does not support legacy architecture
• Move legacy instructions to:
  • README-v3.md
  • README.zh-v3.md
• Add iOS AppDelegate.swift full snippet
• Add Android optional windowIsTranslucent note for reducing visible system icon phase on some devices

Breaking changes

• v4 supports only React Native >= 0.84.0
• Legacy architecture users should stay on v3.x

Validation

• Android example builds and runs successfully
• iOS example builds and runs successfully
• Verified splash show/hide works in the updated example