docs: update read me for mmkv with insturctions for 4.0

[Francesco-Voto]

Description

Update the doc for mmkv plugin in order to include the instructions for mmkv 4.0 as well

---

Note

Updates packages/mmkv-plugin/README.md with separate integration instructions for MMKV <4 (array storages with MMKV) and >=4 (object storages with createMMKV).

• Docs:
  • Update packages/mmkv-plugin/README.md Quick Start integration:
    • Add section for MMKV < 4.0.0 using MMKV and storages: [ ... ].
    • Add section for MMKV >= 4.0.0 using createMMKV and storages: { ... }.
  • Provide version-specific code examples for app integration.

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