I'm not sure what that statement was based on. The stack project certainly still uses haddock to document its code.
At least some of the projects he cites also use readthedocs for user guides etc.
> How do I get (I mean, build in stack) my useful documentation?
Try
stack haddock --open <packagename>
So, what do they use? How do I get (I mean, build in stack) my useful documentation? (Actually I appreciate any tips how to build documentation for off-line use.)