docs(tutorial): Bitcoin Logic - Verifying L1 Transactions with was-tx-mined?

[jadonamite]

Description

This PR adds Module 25: Bitcoin Logic by @jadonamite. It explains the usage of the was-tx-mined? native function to verify Bitcoin transaction inclusion on-chain.

Content Overview

• Merkle Verification: How Clarity validates the path from Tx to Block Header.
• Security: The critical importance of validating the Block Header against the burn chain consensus.
• Data flow: Sourcing proofs from off-chain APIs to on-chain contracts.

Checklist

• Syntax checked for was-tx-mined? tuple signature.
• Header validation pattern included.
• Formatting follows the repository's style guide.

Related Issues

Closes # (If applicable)