English docs and passwordless SSH mesh for lab fleet

- Translate all documentation to English
- Add docs/ssh-mesh.md and scripts/setup/setup-ssh-mesh.sh
- Add config/hosts/atc-lab.hosts for /etc/hosts on all VMs
- Ignore atc_cluster private key in git; mesh deployed on 12 hosts

scripts/setup/README.md

@@ -0,0 +1,7 @@
+# Setup scripts
+
+| Script | Purpose |
+|--------|---------|
+| `setup-ssh-mesh.sh` | Deploy passwordless SSH cluster key to all lab VMs |
+
+Run from Proxmox as root after cloning this repo. Requires existing hypervisor SSH access to targets.