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

.gitignore

@@ -2,8 +2,10 @@
 config/homepage/logs/
 *.log
 
-# Secrets (lokaal op host, niet committen indien je .env gebruikt)
+# Secrets (keep on host; use .env.example in git)
 .env
+**/atc_cluster
+!**/atc_cluster.pub
 .env.local
 .env.*.local