docs: add end-user guide for PrivacyLayer users

[Emins25]

Closes #20

Summary

• Added docs/USER_GUIDE.md focused on non-technical users
• Covers user flow end-to-end: setup, deposit, withdrawal, note management, privacy limitations, troubleshooting, and FAQ
• Included visual aids (Mermaid diagrams) for deposit and withdrawal flow
• Updated README to include a direct link to the user guide

What Is Included

• Beginner-friendly explanations of privacy guarantees and limitations
• Practical step-by-step checklists for deposit and withdrawal
• Operational security guidance (address separation, timing hygiene, note handling)
• Troubleshooting mapped to contract-level error semantics (e.g. UnknownRoot, InvalidProof, NullifierAlreadySpent)
• A concise safe-use checklist for day-to-day operation

Files Changed

• docs/USER_GUIDE.md
• README.md

Notes

• Content references current contract behavior from:
  • contracts/privacy_pool/src/contract.rs
  • contracts/privacy_pool/src/types/errors.rs
  • contracts/privacy_pool/src/core/deposit.rs
  • contracts/privacy_pool/src/core/withdraw.rs