[nrf fromlist] doc: ipc: Add detailed protocol specs for ICBMsg backend

The details of how the ICBMsg communication is done, are hidden
inside the source code. This specification adds details about
the ICBMsg protocol.

Upstream PR: zephyrproject-rtos/zephyr#77229

Signed-off-by: Dominik Kilian <[email protected]>

Author: doki-nordic