`nixos-rebuild` now works perfectly with flakes so this is no longer
required. A new command `build-iso` is still available so that
building a NixOS image is still possible.
Keep the README.md lean and use DOC.md for more detailed explanations. While
revising user documentation, a default `nixos` user profile was created, which
serves as a goood base example.
`nixos-rebuild` has unstable flake support. By adding a simple rebuild
command to the shell, we don't need to reply on it to switch
configurations anymore. At least until it stablizes.
