docs: expand paid MCP tool guide

[mateoalifraco]

Summary

• expand the paid MCP tool example into a practical, reproducible guide
• add a Next.js-style x402 paid endpoint example with HTTP 402 challenge output
• document catalog metadata, preview-tool shape, llms.txt/agents.txt discovery, and a launch checklist

Bounty

MYA/Pyrimid job #24: paid MCP tool guide ($10 USDC)

Payout

Base USDC: 0x97620eB52A577cabc95911099E88e83115eB7C20

Verification

• git diff --check
• secret scan over diff: no secrets/tokens/keys