[Foundations > Merkle proofs] Add guide of how the proof validation logic works.

[aigerimu]

• Merkle Proofs: Complex cryptographic validation in Tact/FunC is hard to implement from scratch.
• Advanced Guide: Add to a "Cryptography" section with a breakdown of how the proof validation logic works.
• https://github.com/tact-lang/dex/blob/main/sources/contracts/vaults/proofs/block-proof.tact