nixos-generate-config: Include extraGroups "wheel"

I've been asked, on numerous occasions, by my students and others, how
to 'sudo' on NixOS.

Of course new users could read up in the manual on how to do that, or we
could make it more accessible for them by simply making it visible in
the default `configuration.nix` file.

Additionally, as raised in [1], replacing `guest` with something more
recognizable could be potentially beneficial to new users. I've
opted for `jane` for now.

[1]: https://github.com/NixOS/nixpkgs/pull/54519#issuecomment-457012223