Add watchdog docs

docs/u_e-watchdog-thresholds.md

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+Watchdog has default values for all thresholds which fit great for most of setups.
+
+Thresholds variables don't added to `mailcow.conf` by default.
+To adjust them just add needed threshold variable to `mailcow.conf` and run `docker-compose up -d`.
+
+## Thresholds description
+
+### MAILQ_CRIT and MAILQ_THRESHOLD
+
+Notificaty administrators if number of emails in the postfix queue is greater then `MAILQ_CRIT` for periond of `MAILQ_THRESHOLD * (60±30)` seconds.