Java API Documents Linking (#1367)

* Show linking against Javadoc

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

* Add sbt-api-mappings plugin

This adds the sbt-api-mappings plugin which enables
auto-linking (properly setting the apiMappings for the sbt project) so
that Scaldoc/unidoc generation will now automatically link against
Java and Scala API docs (and use the right version).

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

* Show linking against Scala APIs

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