[Property Editor] Link to docs

Splitting of from https://github.com/flutter/devtools/issues/9034 to address:

> Should we add a link to the widget’s API reference? This would let the user read things like the widget properties’ docs.

Need to confirm if the Flutter docs URLs are consistent, but we might be able to create this with:

`https://api.flutter.dev/flutter/widgets/{{WIDGET NAME}}-class.html`

e.g. https://api.flutter.dev/flutter/widgets/ListView-class.html