installer-bootstrap: remove unneeded doc options, enable NetworkManager

[normalcea]

We import 3 files in the bootstrap-installer

  imports = [
    (modulesPath + "/profiles/minimal.nix")
    (modulesPath + "/profiles/installation-device.nix")
    (modulesPath + "/installer/cd-dvd/iso-image.nix")
  ];

In nixos/modules/profiles/minimal.nix

  documentation = {
    enable = mkDefault false;
    doc.enable = mkDefault false;
    info.enable = mkDefault false;
    man.enable = mkDefault false;
    nixos.enable = mkDefault false;
  };

We had duplicated this in our configuration, so we can safely remove it.

In nixos/modules/profiles/installation-device.nix

    # Provide networkmanager for easy network configuration.
    networking.networkmanager.enable = true;

NetworkManager is used by default in the minimal ISO and in my initial testing, connecting to the network works fine but requires the user to use sudo or be root which I can't find the root cause of.

The ARM-compiled installer gets an increase from the networkmanager addition from 529MB -> 546MB (~17MB) on my testing. I have not checked the difference on the cross-compilation.