[nrf fromtree] 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.

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

(cherry picked from commit 8d99393)
(cherry picked from commit 567fe96)

Author: doki-nordic