[Docs] API Reference — Full documentation for all contract entry points and SDK methods

[ryzen-xp]

📝 Description

Write the complete API reference documentation for Alien Gateway, covering all Soroban contract entry points and all TypeScript SDK methods. This documentation will live in docs/ and be linked from the README.

✅ Requirements

• Document all public contract entry points: submit_proof, resolve, add_stellar_address, add_chain_address, set_memo, create_vault, deposit, withdraw, schedule_payment
• Document all SDK methods: resolveUsername, sendToUsername, fetchProxyAddress, verifyOwnership
• Include input/output types, error codes, and example calls for each
• Markdown format, clear headings, and code examples

🎯 Acceptance Criteria

• All contract entry points documented with inputs, outputs, errors
• All SDK methods documented with TypeScript usage examples
• docs/api-reference.md exists and is linked from README.md
• No broken links or code examples that do not match actual implementation

📁 Expected files to change/structure

• docs/api-reference.md — new file
• README.md — add link to docs

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🌿 Branch: docs/api-reference | Priority: LOW | Difficulty: ☕ one-coffee