Document ethernet cage leds behaviour for all our boards
This should add a table with status indication LEDs explanation.
- On 1.x we have non-standard behavior so we should describe it for sure
- Omnia has no ethernet cage LEDs and it should be noted that front leds are used instead (blinking on activity and light on link up, there is no standard way to indicate speed)
- Mox behavior is being discussed in turris/os/build#272