Add End-User Documentation and Guide (Issue #20)

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Summary

Implements Issue #20 — Write End-User Documentation and Guide for PrivacyLayer.

What was added

**** — A comprehensive, 1,500+ word user guide covering:

• Introduction — What PrivacyLayer is, how ZK-based privacy pools work, guarantees and limitations
• Getting Started — Freighter wallet setup, funding, connecting to the dApp
• Making a Deposit — Step-by-step deposit flow, note generation, backup critical warning
• Making a Withdrawal — Step-by-step withdrawal flow, proof generation, timing recommendations
• Note Management — How notes work, secure backup methods, recovery (impossible — emphasized)
• Security Best Practices — Wallet security, note security, withdrawal privacy tips
• Privacy Considerations — Anonymity set, timing attacks, amount correlation, on-chain observers
• Troubleshooting — Common errors and solutions, transaction stalled guide
• FAQ — 9 frequently asked questions covering all critical aspects

Acceptance criteria met

• Complete file ✅
• 1,500+ words (≈ 1,600 words) ✅
• All 9 sections from the issue spec ✅
• Security warnings prominently placed ✅
• FAQ section comprehensive ✅
• Non-technical language throughout ✅

Bounty

• Label: / / /
• Reward: USDC via Drips Wave

Closes #20