add nixos flake example

[KiaraGrouwstra]

initially, going over the list of examples it felt not immediately obvious to me that arion can combine the flake/nixos examples to allow containerizing nixos configurations with packages pinned thru a flake. i feel it would be helpful to maybe add an example demonstrating this ability. for reference, i made a small repo showing how to combine these.

[roberth]

That's nice. We could definitely link to interesting examples in the introduction; I think that's what you're referring to? Perhaps have a section for community examples here?

[KiaraGrouwstra]

sure, that sounds great! 😌