docs: firmware rollouts and backups

Neither area had a page, and both changed materially in v0.24.2/v0.24.3.

The firmware page records why the pipeline downloads and verifies before
rebooting, and what raising "at once" actually costs: halting becomes less
precise, devices reboot together, and the upstream warning is conservative
enough that its absence is not proof of safety.

The backups page leads with the fact that a backup is /export text rather
than a binary blob, since that is what makes preview and diff possible and
what limits what a restore can do. It also states plainly that a backup and
its Config History snapshot are one artifact, so deleting one removes the
other's rollback.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

mkdocs.yml

@@ -65,6 +65,8 @@ nav:
       - Alerting: alerting.md
       - SSH keys: ssh-keys.md
       - Bulk commands: commands.md
+      - Firmware rollouts: firmware.md
+      - Backups: backups.md
   - Devices:
       - Cellular (LTE): cellular.md
       - CAPsMAN: capsman.md