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/)

docs/getting-started/prerequisites.md

@@ -0,0 +1,63 @@
+# Prerequisites
+
+## Required Tools
+
+Ensure you have the following tools installed on your local machine:
+
+| Tool | Purpose |
+|------|---------|
+| `nix` | Package manager with flakes enabled |
+| `sops` | Secret encryption/decryption |
+| `age` | Encryption backend for sops |
+| `ssh` | Remote access |
+
+### Installing Nix
+
+Follow the [official Nix installation guide](https://nixos.org/download/).
+
+Enable flakes by adding to `~/.config/nix/nix.conf`:
+
+```
+experimental-features = nix-command flakes
+```
+
+### Installing Other Tools
+
+With Nix:
+
+```bash
+nix-shell -p sops age
+```
+
+Or install globally via home-manager or system configuration.
+
+## Repository Access
+
+Clone the repository:
+
+```bash
+git clone https://git.cryodev.xyz/steffen/cryodev-server.git
+cd cryodev-server
+```
+
+## Development Shell
+
+Enter the development shell with all required tools:
+
+```bash
+nix develop
+```
+
+## Verifying Setup
+
+Check that the flake is valid:
+
+```bash
+nix flake check
+```
+
+Build a host configuration (dry run):
+
+```bash
+nix build .#nixosConfigurations.cryodev-main.config.system.build.toplevel --dry-run
+```