doc(installtaion): add documentation for flake inputs installation

[aster-void]

https://docs.helix-editor.com/package-managers.html#nixos

the use-site flake will look like this:

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    helix.url = "github:helix-editor/helix/master";
    helix.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = { nixpkgs, ... } @ inputs:
  let
    system = "x86_64-linux";
    pkgs = nixpkgs.legacyPackages.${system};
  in {
    devShells.${system}.default = pkgs.mkShell {
      # use `inputs.helix.packages.${system}.default` as the package along with other packages from `pkgs`,
      # such as in nixos's `environment.systemPackages`, home-manager's `home.packages`, and devShell's `packages` (this example).
      packages = [
        inputs.helix.packages.${system}.default
      ];
    };
  };
  # how to add helix to substituters? copy-pasting helix flake's nixConfig doesn't seem to work...
  nixConfig = {};
}

also, it might be clearer to name this Nix, since using Nix isn't limited to NixOS.