Document the usage of healthdog/watchdogs in the Supervisor

[20k-ultra]

The Supervisor has a /v1/healthy endpoint used for performing healthchecks. This endpoint is used in the following known places:

• balenaOS systemd service file
• Supervisor's Dockerfile
• Device diagnostic scripts

Slight confusion is drawn to the fact that the engine and systemd are monitoring the Supervisor. Are these both required ?

Internal discussion: FD thread

[20k-ultra]

We should try to document the reasoning for the settings used in the watchdogs too. For example the systemd one has values such as:

Restart=always RestartSec=10s WatchdogSec=180

and the Dockerfile:

--interval=5m --start-period=1m --timeout=30s --retries=3