enables groups in scaladoc
enables groups in scaladoc

We've reached the point when we do need to introduce logical groups
into the documentation or otherwise people are going to go crazy.

Rough grouping could include: 1) case class boilerplate (adt/ast infrastructure),
2) attribute apis, 3) type apis, etc according to api groups in various Api.scala files.

However I'm leaving grouping itself for future work, because that's
going to take a while.

scalameta/scalameta,DavidDudson/scalameta,xeno-by/scalameta,Dveim/scalameta,scalameta/scalameta,olafurpg/scalameta,DavidDudson/scalameta,xeno-by/scalameta,olafurpg/scalameta,scalameta/scalameta,MasseGuillaume/scalameta,xeno-by/scalameta,scalameta/scalameta,MasseGuillaume/scalameta,olafurpg/scalameta,scalameta/scalameta,MasseGuillaume/scalameta