RFC: aggregated api-javadoc/scaladoc task

[Sailsman63]

#1688 exposes what seems to be a gap in basic services. How shall a mill build create aggregated documentation for the entire project? Per-artifact docJars are only useful when pulling the artifact into an IDE that uses them.

• This should be a built-in, not a third-party extension
• (Personal preference) The resulting doc should not be packed in a .jar - again, that's only for the IDE that's pulling in an artifact.
• Not clear where this should be implemented, but at most it should only take a single import to make it available to the user.
• Should there be separate tasks for JavaModules and ScalaModules, or should a single task build both? Ideally, they'd be able to cross-reference each other, but I don't know if the doc tools make that possible at all.

Thoughts and feedback welcome. I'm very new to scala, (Really only interested in mill as a build tool for my java projects) so don't be surprised if I ask silly/basic questions.

[lihaoyi]

Should be fixed by #2448, which introduces mill.scalalib.UnidocModule. Should land and go out in the next release