docs: add monitoring guide for libguides-notifier jobs

Co-authored-by: aider (openai/gpt-5) <aider@aider.chat>

docs/monitoring.md

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+# Monitoring libguides-notifier jobs
+
+Quick checks
+- Next and last run times (look for libguides-notifier.timer):
+  systemctl --user list-timers --all
+- Timer and service health:
+  systemctl --user status libguides-notifier.timer libguides-notifier.service
+- Recent run output (stdout/stderr from main.py):
+  journalctl --user -u libguides-notifier.service -n 200 --no-pager
+- Follow logs live:
+  journalctl --user -u libguides-notifier.service -f
+
+Manual operations
+- Run immediately (without waiting for the timer):
+  systemctl --user start libguides-notifier.service
+- Enable or disable the timer at login:
+  systemctl --user enable --now libguides-notifier.timer
+  systemctl --user disable --now libguides-notifier.timer
+- Reload user units after editing *.service/*.timer files:
+  systemctl --user daemon-reload
+- Inspect the effective units systemd is using:
+  systemctl --user cat libguides-notifier.service
+  systemctl --user cat libguides-notifier.timer
+
+Notes
+- The timer uses Persistent=true, so if a scheduled time was missed while the laptop was off, one catch-up run will happen when the machine is back on.
+- All print() output from main.py appears in journalctl for libguides-notifier.service.
+- Time zone for scheduling is set in the timer unit: Timezone=America/New_York.