[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

Start in the Property Editor entry point and verify whether Flutter widget names map consistently to the proposed api.flutter.dev URL pattern. Confirm the URL behavior for a widget such as ListView, then ensure the property editor exposes a working link to that widget's API reference.

Scritto dal modello di indicizzazione a partire dal testo della issue.