Better surface API docs in pub.dev

Some users have said that they like Golang package site [pkg.go.dev](https://pkg.go.dev/), where API docs are integrated with the site and presented front and center (example: [http](https://pkg.go.dev/net/http)).

We can learn from the site and organize Flutter/Dart package documentation better.

Before taking any actions, we'd like to hear community members' thoughts on what they like about the Golang package site. Please comment on this issue, if you have anything to share. :)