Add SD image pipeline, documentation overhaul, and fix module issues

- Add automatic SD image builds for Raspberry Pi via Forgejo Actions
- Enable binfmt emulation on cryodev-main for aarch64 cross-builds
- Add sd-image.nix module to cryodev-pi configuration
- Create comprehensive docs/ structure with installation guides
- Split installation docs into: first-install (server), reinstall, new-client (Pi)
- Add lib/utils.nix and apps/rebuild from synix
- Fix headplane module for new upstream API (tale/headplane)
- Fix various module issues (mailserver stateVersion, option conflicts)
- Add placeholder secrets.yaml files for both hosts
- Remove old INSTRUCTIONS.md (content moved to docs/)

hosts/cryodev-pi/default.nix

@@ -1,5 +1,6 @@
 {
   inputs,
+  lib,
   outputs,
   ...
 }:
@@ -10,12 +11,21 @@
     ./hardware.nix
     ./networking.nix
     ./packages.nix
+    ./sd-image.nix
     ./services
     ./users.nix
 
     outputs.nixosModules.common
     outputs.nixosModules.nixvim
+    outputs.nixosModules.sops
   ];
 
+  # Allow unfree packages (netdata has changed to gpl3Plus ncul1 license)
+  nixpkgs.config.allowUnfreePredicate =
+    pkg:
+    builtins.elem (lib.getName pkg) [
+      "netdata"
+    ];
+
   system.stateVersion = "25.11";
 }