Add docs for "Add a module" feature to generate a new Target Module

Problem/Motivation

The existing docs related to setting up a new Target module are pretty sparse. I believe they were written before the "Add a module" feature existed. This feature is often overlooked, so having some docs to describe it would help.

Proposed resolution

Add a new HOWTO (at least) to describe use of the "Add a module" feature. Perhaps some Background info on Target Modules more generally, and some cross-linking from the "Add Config Enforce to a project" howto, and possibly others (eg. Tutorials).