* add sample code for AnimatedContainer

* use stateful_widget_scaffold snippet template

* add sample explanation

* add setState() to example assumptions for analyzer

* update description

* use snippet instead of sample

* change sample to match assets-for-api-docs

* remove constant

* update AnimatedContainer sample description

add indication the example is depicted by the video and animates when
tapped

* fix example formatting

* make sample conform to 80 col limit

* add "implemented below" to illustration description

* formatting for readability