NextGen modules documentation and examples

Since the first post about the NextGen modules some improvements have been done and some design choices have been done (the most important one is the params_lookup pattern which is the result of some good feedback and makes it possible to automatically retrieve class' arguments from top scope variables, an ENC and / or Hiera).

Here is some detailed documentation on the common arguments of the new modules:

And to make things, hopefully, clearer, here some sample usage code snippets: