docs: add x402 payment standard page under AI Studio on Hedera

[piotrswierzy]

Description:

Add a top-level x402 doc covering Hedera's official "exact" payment
scheme and reference implementation in the x402 repo.

• Explain what x402 is/is not, who uses it, and its use cases to correct the "magic crypto-for-agents" misconception
• Document the facilitator-as-feePayer flow and the PaymentRequirements /PaymentPayload/SettlementResponse messages
• Add client/server usage examples; note @x402/hedera is not yet on npm (use from the x402 monorepo source)
• Cover requirements and limitations
• Highlight the Blocky402 facilitator (verified hedera:testnet endpoint and docs) plus how to run and get listed as a Hedera facilitator
• Link the Hedera blog, scheme spec, package, whitepaper, and report

Register the page in docs.json under the "AI Studio on Hedera" group.

Related issue(s):

Fixes #

Notes for reviewer:

Checklist

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

[chopmob-cloud]

@piotrswierzy @walkerlj0 -- adding a data point for the Hedera x402 facilitator listing (point 6 of the issue scope on hashgraph/hedera-agent-kit-js#806).

AlgoVoi APM (Agent Payment Module) runs x402 on Hedera in production via the gateway at api.algovoi.co.uk. The Hedera-specific envelope shape was posted on the parent issue #806 on 2026-05-16 as a concrete example of the exact scheme: HTS token-ID asset field, Hedera account-ID payTo field, fee-payer model. Two shape gotchas worth a callout in the page (HTS decimal-string asset, not 0x hex; Hedera 0.0.X account-ID payTo) are also in that comment.

Facilitator surfaces:

• Production endpoint: api.algovoi.co.uk -- supports the Hedera exact scheme alongside 7 other chain families (Algorand, VOI, Solana, Stellar, Base, Tempo, Ethereum)
• Hedera testnet x402 resource example: https://api.algovoi.co.uk/r/demo/x402-test-hedera
• /supported declaration includes Hedera mainnet + testnet with the standard signer metadata
• Settlement emits categorical receipts under draft-hopley-x402-canonicalisation-jcs-v1 (POSTED IETF Internet-Draft, sole AlgoVoi authorship); the receipt-format suite covers compliance / settlement / cancellation / refund / composite-trust-query
• APM landing page: docs.algovoi.co.uk/platform/apm
• Substrate Adopters Registry recording the canonicalisation discipline: docs.algovoi.co.uk/adopters

The page would benefit from at least two production-grade Hedera facilitator entries (parallel coverage, not single-vendor) so readers see Hedera x402 as ecosystem infrastructure rather than implementation-specific. Happy to PR the facilitator-list entry directly against this branch or file a follow-up PR once #550 lands -- whichever path keeps the review cycle short for you.

-- AlgoVoi