SI-9006 Scaladoc: explicit companion and package links

The existing navigation mechanisms have proved hard to discover for newcomers
to Scaladoc.

This commit adds textual links in the navigation bar to the docs of the
companion (if defined) and to those of the enclosing package.
Merge pull request #4165 from dickwall/ticket/9006

SI-9006 Scaladoc: explicit companion and package links