doc: getting_started: Windows MSYS2 with CMake
doc: getting_started: Windows MSYS2 with CMake

Update the Windows MSYS2 instructions with the required CMake commands
used to build on this platform.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>

aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,nashif/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,galak/zephyr,zephyriot/zephyr,explora26/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,punitvara/zephyr,finikorg/zephyr,mbolivar/zephyr,explora26/zephyr,aceofall/zephyr-iotos,kraj/zephyr,zephyriot/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,ldts/zephyr,finikorg/zephyr,Vudentz/zephyr,mbolivar/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,explora26/zephyr,aceofall/zephyr-iotos,galak/zephyr,kraj/zephyr,kraj/zephyr,galak/zephyr,mbolivar/zephyr,nashif/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,explora26/zephyr,kraj/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,ldts/zephyr