toolchain: doc: Custom toolchain doc updated for other.h requirement
toolchain: doc: Custom toolchain doc updated for other.h requirement

Added missing toolchain name to path for generic.cmake and target.cmake.

Added section explaining the need for, and the use of, the
toolchain/other.h header file.

Signed-off-by: Danny Oerndrup <59721fa3a0e16aab42707531604e65b18fe7db4c@demant.com>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr