Make example path for NixOS install consistent

[ThibaultLemaire]

Yeah I'm the kind of guy that copy-pastes stuff. And this one didn't work :(

[lf-]

Hi! I'm not sure if I understand the full context of this change. It looks like it's moving the code to the parent directory of the programs directory. I'm unfamiliar as to the implications of such though (this section of the readme was contributed, and I don't personally use home-manager). Can you clarify how the directory layout works?

Do we still need to provide that style of install information now that nix-doc is in nixpkgs and is otherwise in maintenance mode, from my end (I don't have any planned features and don't know of any bugs; it just works)?

Thanks for your contribution!

[ThibaultLemaire]

nix-doc is in nixpkgs

Oh it is 😮 ? Silly me I found your project here and didn't check 🤦‍♂️

If you're okay with removing that part of the documentation, I can change this PR to replace it with an example of using it from nixpkgs (i.e. nix-shell --packages nix-doc --run "nix-doc callPackage"). (Although that command doesn't work on my end, but I've opened #14 about that)

Can you clarify how the directory layout works?

There's no convention here. It's just that the previous section of the README says to put a derivation file in /etc/nixos/program/nix-doc/default.nix (where the program/nix-doc/ part is just arbitrary). But then the following section gives example code of how to call this custom derivation from the /etc/nixos/configuration.nix (note that this is the system configuration, nothing to do with home-manager here, although it would work the same). This is where the inconsistency was: the system configuration was trying to call a package located at /etc/nixos/../program/nix-doc/ == /etc/program/nix-doc/ and I got an error that it couldn't find the file.

But if it's in nixpkgs anyway I vote we just delete that part.