Pull request overview

This PR adds comprehensive documentation for the x402 payment protocol on Stellar, including asset configuration details, step-by-step setup instructions, and troubleshooting guidance.

Changes:

• Added "Supported Assets" section documenting testnet and mainnet USDC configuration with issuer addresses and SEP-41 contract details
• Added detailed "Step 3: Establish USDC Trustline" section with both Stellar Laboratory UI instructions and JavaScript code example for creating trustlines
• Added "Troubleshooting" section with common error messages and debugging tips using curl commands
• Added "Understanding Soroban Authorization" section explaining how x402 authorization entries work on Stellar

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/build/agentic-payments/x402/README.mdx	Added "Supported Assets" section with testnet and mainnet USDC details in table format
docs/build/agentic-payments/x402/quickstart-guide.mdx	Added detailed Step 3 trustline setup instructions (UI and code), updated step references, and added troubleshooting section with common errors and debugging tips
docs/build/agentic-payments/x402/built-on-stellar.mdx	Added "Understanding Soroban Authorization" section explaining the authorization entry model and its benefits

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