docs: fix x402 protocol broken links

[peterxing]

Summary

• Replace the broken x402 Fundamentals CTA with the existing xpay Facilitator page.
• Replace adjacent dead links to First Payment and Integration Patterns with existing docs pages.
• Remove the arrow glyphs from the three quick-start link labels so the section stays plain and consistent.

Fixes #3.

Verification

• npm run build
• Live checks before the patch confirmed /en/x402-protocol/fundamentals, /en/x402-protocol/integration-patterns, and /en/getting-started/first-payment currently return 404.
• Live checks confirmed /en/x402-protocol/facilitator, /en/getting-started/installation, and /en/getting-started/production-deployment are valid docs pages.

[vercel]

Someone is attempting to deploy a commit to the Agentically's projects Team on Vercel.

A member of the Team first needs to authorize it.

[peterxing]

I fixed the reported broken x402 link path and adjacent stale links here.

If useful, I can do the broader fixed-scope x402 docs/readback packet for A$690: stale spec links, README examples, facilitator references, first-payment smoke test, and agent spending-control docs.

Details/proof/payment options: https://0741ec59.farmbot-platform-mvp.pages.dev/hire-agent/

Default payment is USDC on Base or Polygon, or invoice if preferred.