Missing `HeroController` inheritance

[benPesso]

According to this article, if you're not using Navigator 1.0, Heros will essentially not work, as they rely on a HeroController provided by Navigator to conduct their transitions.

Took me a good while to figure out why Hero animations were not working with GoRouter, until I accidently stumbled on this little gem of an article - which I doubt many others are familiar with - and managed to get it working.

By wrapping with a HeroControllerScope widget:

```dart MaterialApp( home: HeroControllerScope( controller: HeroController(), child: Router( routerDelegate: _router.routerDelegate, routeInformationParser: _router.routeInformationParser, ), ), ... ); ```

Or...

by adding a HeroController observer:

```dart _router = GoRouter( observers: [HeroController()], ... ); ```

Ideally, GoRouter should include a HeroController observer by default (with option to override, for tween behavior modification). At the very least, the GoRouter > Transitions section should probably mention something about Hero animations and how to get them working with GoRouter.

[csells]

Cool! Would you like to propose a new sample that supports Hero graphics in a PR along with some updates to the docs?

[benPesso]

Yeah sure. Just need to iron out some kinks with Hero animation jitter first. 😶

[csells]

closing this due to lack of customer input. feel free to reopen it if you have a minimal repro.