[Modules] Improved documentation for module collections

[Xavier D. <xav...@gm...>]

Hello All,

Following a request made here to have more insight of what can be done
with module collections, the documentation of this feature has been
improved with examples:

https://modules.readthedocs.io/en/latest/module.html#collections-1

With the introduction of "Initial environment state" and "Stash
collections", it was also the good time to extend this documentation.

Regards,
Xavier