docs(fxrp): add x402 payment protocol guide (#1139)

Author: 0xreflexivity

docs/fxrp/overview.mdx

@@ -65,6 +65,12 @@ Guides for working directly with the FXRP token.
       href: "/fxrp/token-interactions/usdt0-fxrp-swap",
       docId: "fxrp/token-interactions/usdt0-fxrp-swap",
     },
+    {
+      type: "link",
+      label: "x402 Payment Protocol",
+      href: "/fxrp/token-interactions/x402-payments",
+      docId: "fxrp/token-interactions/x402-payments",
+    },
   ]}
 />