nixos/doc: push all the `enable*' and `package*` options to the top of their option group

Why? Because this way configuration.nix(5) can be read linearly.

Before:

> virtualisation.xen.bootParams
> ...
> virtualisation.xen.enable
> ...
> virtualisation.xen.package
> ...

After:

> virtualisation.xen.enable
> virtualisation.xen.package
> virtualisation.xen.bootParams
> ...