Make API docs URLs more consistent with Dart/Flutter SDKs

[jakemac53]

For dart, the docs are at api.dart.dev. For flutter, they are at api.flutter.dev, but for pub, they are at pub.dev/documentation. I can never remember the pub link though and end up just going to the regular pub page for a package and clicking docs.

I would love it if api.pub.dev was its own landing page just for package docs, with a search functionality (and even recent docs you visited would be amazing, just from cookies or some such). And then appending /<package> would take you right to the docs for that package.

This would make the URLs more consistent, shorter, and I believe also more discoverable. We could also keep the existing URL but make the subdomain redirect to that?