Add banner on discontinued packages on dartdoc pages

Pub.dev supports [marking a package as discontinued](). When this happens the main pub.dev UI shows a prominent red [DISCONTINUED] badge:

However, the generated API docs pages don't show any such indication. As these pages are still hosted (to allow for potential continued use), developers might stumble into them. We should add an indication on these pages; I suggest a banner right below the top bar before the API docs themselves: