check-health: monitor free RAM

---- ✂️ ----
🗃️📉️ Health warning: free RAM

The available free RAM on MikroTik is at 18% (47MiB)!
---- ✂️ ----
🗃️📈️ Health recovery: free RAM

The available free RAM on MikroTik increased to 65% (168MiB).
---- ✂️ ----

doc/check-health.md

@@ -13,14 +13,16 @@ This script is run from scheduler periodically, sending notification on
 health related events:
 
 * high CPU load
+* low available free RAM
 * voltage jumps up or down more than configured threshold or drops below limit
 * power supply failed or recovered
 * temperature is above or below threshold
 
 Note that bad initial state will not trigger an event.
 
-Monitoring CPU load works on all devices. Other than that only sensors
-available in hardware can be checked. See what your hardware supports:
+Monitoring CPU load and available free RAM works on all devices. Other
+than that only sensors available in hardware can be checked. See what your
+hardware supports:
 
     /system/health/print;
 
@@ -31,19 +33,24 @@ available in hardware can be checked. See what your hardware supports:
 ![check-health notification cpu load high](check-health.d/notification-01-cpu-load-high.avif)
 ![check-health notification cpu load ok](check-health.d/notification-02-cpu-load-ok.avif)
 
+#### Available free RAM
+
+![check-health notification free ram low](check-health.d/notification-03-free-ram-low.avif)
+![check-health notification free ram ok](check-health.d/notification-04-free-ram-ok.avif)
+
 #### Voltage
 
-![check-health notification voltage](check-health.d/notification-03-voltage.avif)
+![check-health notification voltage](check-health.d/notification-05-voltage.avif)
 
 #### Temperature
 
-![check-health notification temperature high](check-health.d/notification-04-temperature-high.avif)  
-![check-health notification temperature ok](check-health.d/notification-05-temperature-ok.avif)
+![check-health notification temperature high](check-health.d/notification-06-temperature-high.avif)  
+![check-health notification temperature ok](check-health.d/notification-07-temperature-ok.avif)
 
 #### PSU state
 
-![check-health notification psu fail](check-health.d/notification-06-psu-fail.avif)  
-![check-health notification psu ok](check-health.d/notification-07-psu-ok.avif)
+![check-health notification psu fail](check-health.d/notification-08-psu-fail.avif)  
+![check-health notification psu ok](check-health.d/notification-09-psu-ok.avif)
 
 Requirements and installation
 -----------------------------