Add excerpts for snippets on 'Navigation and routing' page

[parlough]

Page URL

https://docs.flutter.dev/ui/navigation/

Page source

https://github.com/flutter/website/tree/main/src/ui/navigation/index.md

Describe the problem

The code snippets on this page don't have excerpts and therefore can easily get out of date or fail analysis in the first place.

Expected fix

Add code excerpts to this page.