docs: improve getting started chapters (#10014)

www/apps/book/sidebar.mjs

@@ -8,12 +8,19 @@ export const sidebar = numberSidebarItems(
       type: "link",
       path: "/learn",
       title: "Introduction",
-    },
-    {
-      type: "link",
-      path: "/learn/first-customizations",
-      title: "Your First Customizations",
-      chapterTitle: "First Customization",
+      children: [
+        {
+          type: "link",
+          path: "/learn/installation",
+          title: "Installation",
+        },
+        {
+          type: "link",
+          path: "/learn/first-customizations",
+          title: "Your First Customizations",
+          chapterTitle: "First Customization",
+        },
+      ],
     },
     {
       type: "link",