Improve Scaladoc package documentation

We will now link to the scaladsl package directly. We should have some documentation pointers there to guide people where to look further. Right now it is empty.

Locate the Scaladoc entry point for the scaladsl package and inspect how nearby package documentation is written. Add documentation pointers that guide readers to further information, then verify that the generated scaladoc package page is no longer empty and the links lead to relevant documentation.

Written by the indexing model from the issue text.