Add Foundry Counter tutorial for Monad testnet

[Phantagyro]

🧪 Tutorial: Deploying a Minimal Counter Contract to Monad (Foundry)

This PR adds a beginner-friendly tutorial for deploying a simple Counter.sol contract to the Monad Testnet using Foundry.

• Creates tutorials/foundry-counter folder
• Adds README, Counter.sol, Deploy.s.sol, and .env.example
• Helps users deploy to Monad using official RPC and their private key

Let me know if you'd like me to extend this into a dApp frontend or verification tutorial as a follow-up.

Thanks!