add documentation

This doesn't work yet for reasons that are not entirely clear to me:

    > nix run .#deploy.all --show-trace
    error: 'nixosConfigurations.all.type' is not a string but

sic!

    > nix build .#installers.all
    warning: Git tree '/home/vg/src/simple-nixos-fediverse' is dirty
    error:
           … while calling the 'derivationStrict' builtin

             at /builtin/derivation.nix:9:12: (source not available)

           … while evaluating derivation 'nixos-24.11.20240815.9286249-x86_64-linux.iso'
             whose name attribute is located at /nix/store/nqqkj0pwx2ymv8rxpw1m80zd4fxkvk0s-source/pkgs/stdenv/generic/make-derivation.nix:334:7

           … while evaluating attribute 'sources' of derivation 'nixos-24.11.20240815.9286249-x86_64-linux.iso'

             at /nix/store/nqqkj0pwx2ymv8rxpw1m80zd4fxkvk0s-source/nixos/lib/make-iso9660-image.nix:76:3:

               75|
               76|   sources = map (x: x.source) contents;
                 |   ^
               77|   targets = map (x: x.target) contents;

           (stack trace truncated; use '--show-trace' to show the full trace)

           error: Could not load a value as a module, because it is of type "flake", in file /nix/store/nqqkj0pwx2ymv8rxpw1m80zd4fxkvk0s-source/flake.nix.

???

flake.nix

@@ -63,9 +63,10 @@
       all = nixpkgs.lib.nixosSystem {
         inherit system;
         modules = with self.nixosModules; [
+          disko
+          disk-layout
           fediversity
           interactive-vm
-          disko
           peertube-vm
           pixelfed-vm
           mastodon-vm

vm/disk-layout.nix

@@ -1,6 +1,7 @@
 { ... }:
 {
   disko.devices.disk.main = {
+    device = "/dev/sda";
     type = "disk";
     content = {
       type = "gpt";