02_agl_compositor: Added gRPC proxy docs

02_agl_compositor: Added gRPC proxy docs

Adds the proto buf protocol as well as pointing out to
agl-shell-activator or window-management-clieng-grpc.

Bug-AGL: SPEC-4833
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ib683c09e81cfbb03c9fdbc290e475c6563de9025
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/29722
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Lisandro Perez Meyer <lpmeyer@ics.com>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>