Add External Configuration Modules

External-Configuration-Modules.md

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+# External Configuration Modules
+
+External modules allow system and user configuration to live in
+separate Git repositories.
+
+## System Modules
+
+Used in `inventory.nix` to define complete host configurations.
+
+```nix
+devices."server" = builtins.fetchGit {
+  url = "https://git.example.edu/org/server-config";
+  rev = "abc123";
+};
+```
+
+## User Modules
+
+Defined in `users.nix` and typically include both account options and
+home-manager configuration.
+
+Each external user module may contain:
+- `user.nix` (required)
+- `nixos.nix` (optional)
+
+## Evaluation Order
+
+1. User NixOS modules
+2. Host type modules
+3. Inventory overrides
+4. External system modules
+
+Later modules override earlier ones.