[Added] improved modem/router watchdog tutorial

[iec61131]

Thanks @arendst for this amazing project.
After a many hours spent I finally created a cookbook recipe for how to turn $8 S31s into WifiRebooters.
During this process I found couple issues which I fixed:

• doc watchdog Rules did not work out of box
• discovered bug of missing unix time which prohibits Tasmato to execute rule without WiFi

Please check my project https://github.com/iec61131/WiFiRebooter
Feel free to merge any of it in the docs etc.

[pkkrusty]

Docs pages are maintained by the community, so things probably won't get changed unless you initiate.