Add React Native setup instructions for encrypted channels

[ttypic]

Description

Based on internal customer feedback: https://ably-real-time.slack.com/archives/C07RGGVHQ7L/p1761225328497749

Add React Native setup instructions for encrypted channels

• Documented the use of WebCrypto APIs (crypto.subtle methods) and additional dependencies for React Native.
• Provided a list of recommended polyfills for cryptographic primitives and text decoding in React Native.

Checklist

• [x] Commits have been rebased.
• [] Linting has been run against the changed file(s).
• [] The PR adheres to the writing style guide and contribution guide.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• [ ] Create PR with unit tests
• [ ] Post copyable unit tests in a comment
• [ ] Commit unit tests in branch react-native-encrypted-channels

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