docs: add MPP (Celo-hosted x402 facilitator) page

[viral-sangani]

Summary

Adds a MPP page under Build with AI → Agent Infrastructure, documenting Celo's hosted x402 facilitator at x402.celo.org.

The existing x402.mdx covers the protocol and the thirdweb-hosted facilitator; this page covers the Celo-native hosted option — so builders who want a Celo-first, ready-to-use facilitator have a direct path. The two pages cross-link each other.

What's included

• High-level explanation of what MPP is (hosted x402 facilitator: seller returns 402, buyer signs EIP-3009, facilitator settles on-chain and pays gas) and why it fits Celo.
• Code examples — facilitator client, seller (@x402/hono), and buyer (@x402/fetch). These mirror the live integration guide and were verified end-to-end against the facilitator on Celo Sepolia (real on-chain settlement).
• Runnable example repo — links to celo-org/mpp-celo-example, a minimal seller + buyer you can clone and run.
• CTA to the agent skill — points integrators at x402.celo.org/skill.md so their AI coding assistant can add MPP payments for them.
• A Celo-specific warning about naming the asset explicitly (bare "$0.01" pricing 500s until coinbase/x402#244 merges).

Changes

• build-on-celo/build-with-ai/mpp.mdx — new page
• docs.json — adds the page to the Agent Infrastructure nav group
• build-on-celo/build-with-ai/x402.mdx — cross-link note to the new MPP page

Validation

• docs.json is valid JSON; the page is registered in the nav.
• All internal links resolve to existing pages; <Note>/<Card>/<Warning> and the icons used are all already in use elsewhere in the docs.
• The seller/buyer code was run against the live Celo Sepolia facilitator and settled a real payment.

[viral-sangani]

Closing this — on review, this page describes the Celo-hosted x402 facilitator but was mislabeled as MPP (Machine Payments Protocol), which is a distinct protocol (Tempo/Stripe). Celo already has x402 documentation, so this page isn't needed. Apologies for the noise.