Add the ergocub battery-pack LED documentation

[valegagge]

We need to complete the battery-pack documentation with the behaviour of the LED that can be retrieved from our internal cloud

Dod
Documentation updated

[MSECode]

Do we still need this? @valegagge

[valegagge]

yes!
Bat before we need to check the LEDs board!!
https://github.com/mesh-iit/electronics-boards/issues/606.

We can remove the parent issue and keep it in this repo, because, in the end, the ergocub S/N 003 hasn't the LEDs working.

[valegagge]

Moved to triage because the LEDs borad doesn't work on ergoCubSN003. We'll updated it when we'll have a led on a battery pack assembled and working.