fix(aio): fix layout of the webpack guide (#18493)

This is possibly a temporary fix for the layout, until we decide whether we want
to remove the guide or properly add it to the SideNav menu.

Fixes #17912

aio/content/navigation.json

@@ -61,6 +61,12 @@
       "hidden": true
     },
 
+    {
+      "url": "guide/webpack",
+      "title": "Webpack: An Introduction",
+      "hidden": true
+    },
+
     {
       "url": "guide/quickstart",
       "title": "Getting Started",