docs: update backup docs (#1082)

docs/backup-and-push-cronjob.md

@@ -9,10 +9,12 @@ services:
     entrypoint: ["bash", "-c"]
     command:
       - |
-        bench backup-all-sites
-        ## Uncomment following to snapshot sites
+        bench --site all backup
+        ## Uncomment for restic snapshots.
         # restic snapshots || restic init
         # restic backup sites
+        ## Uncomment to keep only last n=30 snapshots.
+        # restic forget --group-by=paths --keep-last=30 --prune
     environment:
       # Set correct environment variables for restic
       - RESTIC_REPOSITORY=s3:https://s3.endpoint.com/restic
@@ -52,3 +54,5 @@ Notes:
 - Make sure `docker-compose` or `docker compose` is available in path during execution.
 - Change the cron string as per need.
 - Set the correct project name in place of `erpnext`.
+- For Docker Swarm add it as a [swarm-cronjob](https://github.com/crazy-max/swarm-cronjob)
+- Add it as a `CronJob` in case of Kubernetes cluster.