check/update links to asset documentation once changes to flutter website land

https://github.com/flutter/website/pull/8710 is going to update the [Adding assets and images](https://docs.flutter.dev/ui/assets-and-images). Once this lands, we should take a pass through the relevant code to make sure links still work and have up-to-date titles.