docs: Fix Serialization and ABI Compatibility Update grand_product_ex…

[imbarty]

I've addressed the serialization and ABI compatibility issues in the proof handling logic.
The proof is now properly serialized into bytes using serialize_uncompressed, and the sol! macro defines a struct SolProductProofAndClaims that aligns with Solidity's ABI encoding requirements.
Additionally, I added a helper function fr_to_uint256 to convert field elements to U256 and included error handling with .expect for serialization failures.

Comments have been added to clarify each step for better maintainability.

This should resolve the issues we were seeing with proof encoding and compatibility.