Checks: add nixos documentation-check

checks/nixos-documentation/flake-module.nix

@@ -0,0 +1,24 @@
+{ self, ... }:
+let
+  documentationModule = {
+    # This is how some downstream users currently generate documentation
+    # If this breaks notify them via matrix since we spent ~5 hrs for bughunting last time.
+    documentation.nixos.enable = true;
+    documentation.nixos.extraModules = [
+      self.nixosModules.clanCore
+      { clan.core.clanDir = ./.; }
+    ];
+  };
+in
+{
+  clan = {
+    machines.test-documentation = {
+      # Dummy file system
+      fileSystems."/".device = "/dev/null";
+      boot.loader.grub.device = "/dev/null";
+      imports = [
+        documentationModule
+      ];
+    };
+  };
+}