docs: add wallet implementation guide for wallet_pay standard

[devin-ai-integration]

docs: add wallet implementation guide for WalletConnect Pay

Summary

Adds concise wallet-side implementation documentation for WalletConnect Pay to the Payments section. This guide explains how wallets should handle payment requests that arrive in WalletConnect session proposals.

Changes:

• New page: payments/wallet-implementation.mdx (~220 lines)
• Added to navigation under "WalletConnect Payments Standard"
• Covers SDK integration points, data schemas, and implementation flow
• Simplified to focus on essential SDK interaction patterns without verbose validation/UI code

Technical approach:

• Payment requests arrive via proposal.params.requests.walletPay in session proposals
• Wallets display payment details alongside normal connection approval
• Payment processing is optional - users can approve session with or without payment
• Results returned via proposalRequestsResponses array when calling walletkit.approveSession()

Review & Testing Checklist for Human

• Verify flow accuracy with reference dApp: Test the documented flow by running the reference dApp and connecting with a test wallet to ensure the integration points match reality
• Validate TypeScript types: Check that EngineTypes.WalletPayParams, SignClientTypes, and other type references match the actual published SDK types (not just what's in the example code)
• Verify CAIP format examples: Double-check CAIP-19 asset format (eip155:1/erc20:0x...) and CAIP-10 account format (eip155:1:0x...) examples are correct and the parsing logic shown makes sense
• Test code examples: Ensure the code snippets are clear enough to implement from, especially the pseudo-code sections like parseAsset() and transferToken() which are intentionally left abstract

Notes

• Documentation was simplified significantly based on feedback (from ~600 lines to ~220 lines) to focus on SDK integration rather than implementation details
• Initially misunderstood the flow (thought it was a separate JSON-RPC method call), corrected after reviewing actual code in PR #926
• Code examples are based on analyzing reference implementation but haven't been executed
• Links point to chore/wallet-pay-dapp branch which may be temporary
• Requested by rohit@reown.com (@rohit-710)
• Session: https://app.devin.ai/sessions/abc2d5f5a6ed42819ffcdeacb424d19d

[devin-ai-integration]

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