Added sr25519 migration guide

[ap211unitech]

📝 Description

This PR adds comprehensive documentation for the sr25519 migration from WASM to pure JavaScript implementation (@scure/sr25519), corresponding to polkadot-js/common#1971.

🤔 Why This Documentation is Needed

The SR25519 migration represents a breaking change:

• ❌ Old: Async WASM operations requiring initialization
• ✅ New: Synchronous pure JS operations (no initialization needed)
• Although function signatures remain compatible, the removal of async operations and WASM initialization is a breaking change in behavior. This guide ensures developers can cleanly migrate and take advantage of the performance improvements.

[TarikGul]

Looks fine, I would also mention that the cryptoAwaitReady and can work in conjunction with the non async stuff. Since most usecases might have multiple crypto imports.