Repair advanced feature reference (#10330)

* Repair advanced feature reference

The explanation of the custom document feature had been moved.

* Update README.md

Co-authored-by: Joe Haddad <timer150@gmail.com>

examples/with-google-analytics/README.md

@@ -1,6 +1,6 @@
 # Example app with analytics
 
-This example shows how to use [Next.js](https://github.com/zeit/next.js) along with [Google Analytics](https://developers.google.com/analytics/devguides/collection/gtagjs/). A custom [\_document](https://github.com/zeit/next.js/#custom-document) is used to inject [tracking snippet](https://developers.google.com/analytics/devguides/collection/gtagjs/) and track [pageviews](https://developers.google.com/analytics/devguides/collection/gtagjs/pages) and [event](https://developers.google.com/analytics/devguides/collection/gtagjs/events).
+This example shows how to use [Next.js](https://github.com/zeit/next.js) along with [Google Analytics](https://developers.google.com/analytics/devguides/collection/gtagjs/). A custom [\_document](https://nextjs.org/docs/advanced-features/custom-document) is used to inject [tracking snippet](https://developers.google.com/analytics/devguides/collection/gtagjs/) and track [pageviews](https://developers.google.com/analytics/devguides/collection/gtagjs/pages) and [event](https://developers.google.com/analytics/devguides/collection/gtagjs/events).
 
 ## Deploy your own