Support Java API doc Scaladoc/Unidoc References (#1420) (#1469)

Manually back-ported.

* Link to Java API Documentation

This adds a build.sbt apiMappings to allow for Scaladoc/Unidoc linking
to Java API documentation. This uses the exact strategy that Scala
upstream uses for linking.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

* Add example of Java and Scala API docs linking

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit bdac6e1c

)

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>