doc: clarify where application can use ExternalZephyrProject_Add

The sysbuild documentation refers to CMakeLists.txt which can lead the
user to believe this call can be added to the <sample>/CMakeLists.txt
file.

This file is sourced as part of the Zephyr CMake build and not sysbuild
CMake build.

Therefore change the description to sysbuild.cmake.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>