[Feature Request] Add intermediate step/phase: extract

I would like to consolidate documentation from multiple services/repositories into one site. I don't think this is currently very easy to do. I'd be happy to be wrong, though.

I view it as the following sequence of actions:

1. Generate content from code (`metadata' step).
2. Extract content based on the setting of the corresponding json section.
3. Place the result in some folder in the central doc repo. (in most cases `md` and `yml` files)
4. Build and publish the doc site.

It looks like we are missing the 2nd item from the list.