doc: west: Document build system integration
doc: west: Document build system integration

Document the build system integration with west and any projects managed
by it.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>
Signed-off-by: Torsten Rasmussen <96e313834d43f704020a91f4e5eaf114be1fb11a@nordicsemi.no>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,ldts/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,explora26/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr