docs(ai-agents/payments): page promises network/token info in description but contains none

## Problem

File: docs/ai-agents/payments/pay-for-services-with-x402.mdx

The page's `description` frontmatter explicitly states:

> "How x402 works, how your agent makes payments, and which networks and tokens are supported"

However, the page contains no section on supported networks or tokens. A developer reading this page has no way to know:

- Which chains x402 currently supports (Base, Solana, Arbitrum, Polygon, Ethereum)
- Which tokens are accepted (USDC via EIP-3009, EURC, other ERC-20s via Permit2)
- That the CDP facilitator supports specific networks only

## Expected behavior

Either:
1. Add a "Supported networks and tokens" section to the page matching what the description promises, OR
2. Update the description to remove the unsupported claim

## References

- x402 network support docs: https://docs.cdp.coinbase.com/x402/network-support
- x402 ecosystem: https://www.x402.org/ecosystem