Provide sub documentation pages for populate ecosystem modules

[mglaman]

There is a slight dilemma on what to do for popular contrib modules. We should provide a single page that describes the project but links offsite to the official docs (ie: Drupal.org)

This would allow the user guide to talk about managing stock and link to up to date documentation. Otherwise, we'd have to expand our access to more and more users.

This would be great for the Point of Sale module, which is contrib but also presented as a form of product by Acro Media.