doc: use evaluating instead of iterating

      # is used here as a substitute attribute name for `packages`. The problem

      # with `packages` is that it makes operations like `nix flake show

      # nixpkgs` unusably slow due to the sheer number of packages the Nix CLI

      # needs to iterate through. But when the Nix CLI sees a `legacyPackages`

      # attribute it displays `omitted` instead of iterating through all

      # packages, which keeps `nix flake show` on Nixpkgs reasonably fast,

      # though less information rich.

      # needs to evaluate. But when the Nix CLI sees a `legacyPackages`

      # attribute it displays `omitted` instead of evaluating all packages,

      # which keeps `nix flake show` on Nixpkgs reasonably fast, though less

      # information rich.

      legacyPackages = forAllSystems (system: import ./. { inherit system; });

      nixosModules = {