nixos/release-notes: point `nixos-rebuild build-image` to the right manual (#392306)

- The `nixos-generate-config` command now supports a optional `--flake` option, which will generate a flake.nix file alongside the `configuration.nix` and `hardware-configuration.nix`, providing an easy instroduction into flake-based system configurations.

- A `nixos-rebuild build-image` sub-command has been added.

  It allows users to build platform-specific (disk) images from their NixOS configurations. `nixos-rebuild build-image` works similar to the popular [nix-community/nixos-generators](https://github.com/nix-community/nixos-generators) project. See new [section on image building in the nixpkgs manual](https://nixos.org/manual/nixpkgs/unstable/#sec-image-nixos-rebuild-build-image). It is also available for `nixos-rebuild-ng`.

  It allows users to build platform-specific (disk) images from their NixOS configurations. `nixos-rebuild build-image` works similar to the popular [nix-community/nixos-generators](https://github.com/nix-community/nixos-generators) project. See new [section on image building in the NixOS manual](https://nixos.org/manual/nixos/unstable/#sec-image-nixos-rebuild-build-image). It is also available for `nixos-rebuild-ng`.

- `nixos-option` has been rewritten to a Nix expression called by a simple bash script. This lowers our maintenance threshold, makes eval errors less verbose, adds support for flake-based configurations, descending into `attrsOf` and `listOf` submodule options, and `--show-trace`.