docs: added store how-to guide for categories (#3690)

www/docs/sidebars.js

@@ -630,6 +630,11 @@ module.exports = {
             sidebar_is_soon: true,
           },
         },
+        {
+          type: "doc",
+          id: "modules/products/store/use-categories",
+          label: "Storefront: Use Categories",
+        },
         {
           type: "link",
           href: "#",