nixos-rebuild: Fix quoting in build-image output, improve docs

[phaer]

Don't quote imageName in nixos-rebuild build-image. It's just a string, not actual json. This mixup - which I authored - leads to extra quoting when printing the image name, e.g.:

Done.  The disk image can be found in /nix/store/rynka0dxk4n6c29mggijf8cmbqzf47zh-nixos-image-amazon-25.0
5pre-git-x86_64-linux/"nixos-image-amazon-25.05pre-git-x86_64-linux.vpc"

The rest is just documentation updates

[phaer]

I hope to have addressed all outstanding feedback, so I think this might be good to merge?

[spiage]

broken nixos-rebuild, line 868, " | jq -r . "

        if [[ -z $buildingAttribute ]]; then
            imageName="$(
                runCmd nix-instantiate --eval --strict --json --expr \
                "let
                    value = import \"$(realpath $buildFile)\";
                    set = if builtins.isFunction value then value {} else value;
                in set.${attr:+$attr.}config.system.build.images.$imageVariant.v.passthru.filePath" \
                "${extraBuildFlags[@]}"
        | jq -r .
            )"

❯ nixos-rebuild -v -L switch --flake .
$ cat /proc/sys/kernel/hostname
$ nix --extra-experimental-features nix-command flakes build --out-link /tmp/nixos-rebuild.FQN5AJ/nixos-rebuild .#nixosConfigurations."a7".config.system.build.nixos-rebuild -v -L
$ exec /nix/store/f2zrxs682q0ql7d3llbkgp16580mzac7-nixos-rebuild/bin/nixos-rebuild -v -L switch --flake .
$ cat /proc/sys/kernel/hostname
/nix/store/f2zrxs682q0ql7d3llbkgp16580mzac7-nixos-rebuild/bin/nixos-rebuild: line 868: syntax error near unexpected token `|'

[spiage]

@thiagokokada @phaer @fricklerhandwerk @jsierles