Add CNX Backups dashboard and document the backup setup

Grafana dashboard (auto-provisioned from the dashboards dir) tracks
borgbackup job health, time since last run, and per-job systemd state
from the node_exporter systemd collector on the client. New docs page
covers the ns1 -> control topology, secrets flow, and restore commands.

docs/src/SUMMARY.md

@@ -4,3 +4,4 @@
 - [ZeroTier mesh](./mesh.md)
 - [DNS](./dns.md)
 - [Monitoring](./monitoring.md)
+- [Backups](./backups.md)