Add documentation about Passive DCE Equipment / Passive Devices

[kmpm]

There are a Asset type called "Passive Devices" that seems to be called Passive DCE Equipment in the code.
But there is no documentation on how to use it.
The PR glpi-project/glpi#7118 mentions that it would solve issues with patch panels in racks.

Sure, I can add a passive device as a patch panel in a rack and even create sockets on it. But I don't know how to connect other equipment to those sockets.

Some documentation would be nice. And if I could figure out how it worked I would have been able to contribute to the documentation but I'm kind of lost.