Add cron schedules and sync docs

agent/config.example.toml

@@ -32,3 +32,11 @@ name = "Uptime probe"
 command = "test $(cut -d. -f1 /proc/uptime) -gt 60"
 interval = "60s"
 timeout = "5s"
+
+# Cron-style schedules are also supported. Use exactly one of interval or cron.
+# [[checks]]
+# id = "daily_backup"
+# name = "Daily backup freshness"
+# command = "/usr/local/lib/monlet/check_backup_freshness.sh"
+# cron = "CRON_TZ=UTC 15 6 * * *"
+# timeout = "2m"