Restore link to /intro docs in main menu (#8356)

The "Overview" link previously went to a summary of Consul features and a
comparison to other products. This commit restores that destination at the
request of Consul PMs.

This commit previously removed the link (it went to `/` instead of `/intro`).

ac612a9cdc (diff-f98b55875118725d1373dd2da36d9ee5)

website/data/subnav.js

@@ -1,5 +1,5 @@
 export default [
-  { text: 'Overview', url: '/', type: 'inbound' },
+  { text: 'Overview', url: '/intro' },
   {
     text: 'Use Cases',
     submenu: [