docs: add x402 supported networks and tokens

[peterxing]

Summary

• Add a Supported networks and tokens section to the x402 agent payments guide.
• Clarify facilitator-dependent support for Base, Base Sepolia, Polygon, Arbitrum, World, Solana, and Solana Devnet.
• Explain EIP-3009 tokens, Permit2 ERC-20 payments, and Solana SPL/Token2022 support with a link to the canonical x402 network support docs.

Verification

• git diff --check
• Direct inspection of docs/ai-agents/payments/pay-for-services-with-x402.mdx

Closes #1460

[cb-heimdall]

🟡 Heimdall Review Status

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[peterxing]

Follow-up on the commercial/readback side of this docs PR: I packaged the buyer-readable no-spend scope here:

https://farmbot-platform-mvp.pages.dev/hire-agent/base-x402-network-token-readback/

Scope options:

• A$390 fixed Base x402 network/token docs readback
• A$1,900 docs patch room if the team wants follow-through across adjacent Base/x402 agent-payment pages

What it covers: the supported-network/token claim matrix, EIP-3009 vs Permit2 wording, browser-agent 402/PAYMENT-RESPONSE visibility, facilitator-dependent caveats, and a maintainer-ready docs-drift appendix.

No live paid x402 calls, wallet signing, token approvals, transfers, private credentials, signup, or legal/payment-term acceptance are included. Settlement can be invoice or USDC on Base/Polygon after scope confirmation.