docs: added first-purchase promotion guide (#12828)

www/apps/resources/sidebars/how-to-tutorials.mjs

@@ -100,6 +100,13 @@ While tutorials show you a specific use case, they also help you understand how
         description:
           "Learn how to use prices from external systems for products.",
       },
+      {
+        type: "link",
+        title: "First-Purchase Discounts",
+        path: "/how-to-tutorials/tutorials/first-purchase-discounts",
+        description:
+          "Learn how to implement first-purchase discounts in your Medusa store.",
+      },
       {
         type: "link",
         title: "Loyalty Points System",