A standalone configuration is one that exists in its own project/repository.
This means that you can externalize your entire neovim configuration, including
flake inputs, nixpkgs version, etc...
This decoupling of neovim and NixOS configurations has multiple advantages
including mixing stable/unstable nixpkgs and the ability to run your neovim
configuration outside of your NixOS configuration. You can even run it
outside of NixOS all together.