Add relevant resource links (#2286)

* Add relevant resource links

* Update docs/build/apps/x402.mdx

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update docs/build/apps/x402.mdx

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: janewang

docs/build/apps/x402.mdx

@@ -58,7 +58,17 @@ This version supports x402 v2 specification. For x402 v1 support, please use a p
 - **x402 Starter Template** — A starter template for building payment-enabled applications with x402. Simplified scaffolding demonstrating x402 payment protocol integration with browser wallet support; use it as a foundation for micropayment-enabled services, SaaS applications, or any project that needs frictionless web payments. [View on GitHub](https://github.com/ElliotFriend/x402/tree/stellar-browser-wallet-example/examples/typescript/fullstack/browser-wallet-example)
 - **Economic Load Balancer** — An intelligent multi-chain payment router that automatically selects the most cost-efficient network for high-frequency AI agent micropayments. [View on GitHub](https://github.com/marcelosalloum/x402/tree/x402-hackathon)
 
+## Additional Documentation
+
+- [OpenZeppelin x402 Facilitator Plugin Source Code](https://github.com/OpenZeppelin/relayer-plugin-x402-facilitator)
+- [OpenZeppelin x402 Facilitator Docs](https://docs.openzeppelin.com/relayer/1.4.x/guides/stellar-x402-facilitator-guide)
+- [OpenZeppelin Stellar Relayer SDK](https://github.com/OpenZeppelin/openzeppelin-relayer-sdk)
+- [OpenZeppelin Stellar Relayer Docs](https://docs.openzeppelin.com/relayer/1.4.x)
+- [OpenZeppelin Smart Account](https://docs.openzeppelin.com/stellar-contracts/accounts/smart-account)
+
 ## Learn more
 
 - [x402 protocol (Coinbase Developer Platform)](https://docs.cdp.coinbase.com/x402) — Official x402 protocol overview and spec
+- [x402 protocol specification](https://www.x402.org/) — x402 Specification and Whitepaper
+- [Coinbase x402 GitHub](https://github.com/coinbase/x402) — Official x402 Protocol GitHub Repo
 - [Signing Soroban invocations](../guides/transactions/signing-soroban-invocations.mdx) — Auth-entry signing and transaction signing on Stellar