Update nixos installation guide (without flake / via nixpkgs)

[lalishansh]

Add instructions for installing ags via nixpkgs instead of inputs inside flake.nix
(for people who want to keep flake.nix small and clean 👍🏼 )

  # For aylur-gtk-shell
  home.packages = with pkgs; [
    dart-sass
    bun
    (ags.overrideAttrs (old: {
      buildInputs = old.buildInputs ++ [ pkgs.libdbusmenu-gtk3 ];
    }))
  ];
  # For icon files etc. (inside home.nix)
  gtk = {
    enable = true;
    iconTheme = {
      package = pkgs.papirus-icon-theme;
      name = "Papirus-Dark";
    };
  };
  # Aylur's dotfiles/ags config
  home.packages = with pkgs; [
    libsoup_3
    sassc
    brightnessctl
    fd
  ];