docs: add comprehensive deployment guide

[wocaoac-cpu]

Summary

• Add docs/DEPLOYMENT.md with complete step-by-step deployment instructions for the PrivacyLayer privacy pool contract
• Covers the full deployment lifecycle: prerequisites, circuit compilation, contract compilation, testnet deployment, initialization, configuration, verification, mainnet deployment, and troubleshooting
• Includes security checklist for mainnet deployment and complete error code reference table

Document Structure

1. Prerequisites -- Rust/Cargo, Stellar CLI, Noir toolchain, funded testnet account
2. Circuit Compilation -- Compile all Noir circuits, generate verification keys, export artifacts
3. Contract Compilation -- Build WASM binary, optimize for size, run tests, generate contract hash
4. Testnet Deployment -- Deploy contract, initialize with parameters, verify deployment
5. Configuration -- Admin address, denominations, verifying key updates
6. Verification -- Test deposit, test withdrawal, verify events and contract state
7. Mainnet Deployment -- Security checklist, audit requirements, deployment steps, post-deployment verification
8. Troubleshooting -- Error code table, debug commands, build failure fixes, support resources

Acceptance Criteria

• Complete DEPLOYMENT.md file
• Step-by-step commands for every operation
• Example output for key commands
• Troubleshooting section with error codes and solutions
• Security checklist for mainnet deployment
• All CLI commands use actual stellar contract syntax

Closes #18