Kotlin setup should include Maven & Gradle instructions

In https://github.com/grpc/grpc-kotlin/issues/220#issuecomment-750340662 @jamesward wrote:

> I think we should update [our setup docs](https://grpc.io/docs/languages/kotlin/basics/#setup) to have the Maven & Gradle instructions like here: https://github.com/grpc/grpc-kotlin/tree/master/compiler#build-tool-plugins (Instead of just referencing the examples)