docs: add guide on fulfillment api (#388)

* Create fulfillment-api.md

* fix: sidebar

www/docs/sidebars.js

@@ -77,5 +77,15 @@ module.exports = {
         },
       ],
     },
+    {
+      type: "category",
+      label: "Guides",
+      items: [
+        {
+          type: "doc",
+          id: "guides/fulfillment-api",
+        },
+      ],
+    },
   ],
 }