nix: clean up files, use good practices

[RGBCube]

• nix: clean up files
• nix: update docs to reflect the new setup

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[RGBCube]

I also moved the new nix command to the top, as it should be preferred.

[RGBCube]

Should be good to go then, I've applied the changes requested.

[networkException]

hm I don't like how we're declaring this as "good practice" that are in fact (in practice) considered experimental in Nix. The previous description also stated the difference between the v2 and v3 cli, this does not.

[RGBCube]

hm I don't like how we're declaring this as "good practice" that are in fact (in practice) considered experimental in Nix. The previous description also stated the difference between the v2 and v3 cli, this does not.

The new description mentions that the v2 cli is old and will use the host nixpkgs, I think that's pretty descriptive enough?

Defining flakes as good practice is controversial, but it's the general way the community is going towards as it is more reproducible. Companies also prioritize flake support. (See DetSys, Hercules CI, Garnix and a few more which are all aimed towards flakes)