DITA 2.0: Make all profiling attributes specializations of @props

We’ve talked about this, but it hasn’t made it to the Kanban board on GitHub, so:

DITA 2.0 should refactor the existing profiling attributes – audience, platform, product, and othermeta – so that they’re in their own domain(s) and specialized from @props, as we did for deliveryTarget in
1.3.

Chris