docs: extend configuring section with details on the module system

[NotAShelf]

This is an initial draft of a new modules section describing how we use the Nix module system within nvf. While it does not cover every module, I think it is worth covering the basics of a module to give the users and idea and to potentially answer questions from those New to the Nix ecosystem.

I also think we stand to gain from adding some examples in the DAG section, but I reckon it may be more confusing for the users than it is helpful. CC @horriblename and @Soliprem for reviews and feedback. This can be merged as is, but worth discussing beforehand.

Closes #1067

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