docs: add Ax402 facilitator (x402)

[gsmachado]

Description:

This PR adds Ax402 to the x402 facilitators page, giving developers another hosted facilitator option for Hedera.

• Add an Ax402 section with testnet and mainnet facilitator setup
• Document the open testnet facilitator URL (https://testnet.facilitator.ax402.io) and its advertised fee payer
• Document the mainnet dedicated facilitator URL (https://<subdomain>.facilitator.ax402.io) and the steps to obtain it from the Ax402 dashboard
• Link the Ax402 platform features around the facilitator (gateway, seller dashboard, SDKs, CLI, MCP, paywall)

Notes for reviewer:

Testnet setup requires no account; the mainnet facilitator uses a dedicated per-account URL created through the Ax402 dashboard. Advertised fee payers were taken from each facilitator's public /supported endpoint, and both were verified live.

Checklist:

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[gsmachado]

Hey team!
Could someone with write access assign someone to this PR? That's why the "PR Formatting / Assgnee Check" is failing, and I can't do much.

FYI, @theekrystallee