docs(semaphore): add cleanup playbook reference to deploy playbook

ansible/playbooks/deploy_semaphore.yml

@@ -1,4 +1,8 @@
 ---
+# Cleanup old deployments first with:
+#   ansible-playbook -i inventory.yml playbooks/cleanup_semaphore.yml --limit figment
+#   (add -e "semaphore_force_clean=true" to also remove data)
+
 - name: Deploy Semaphore on figment
   hosts: figment
   become: true