doc: getting_started: Document building with west
doc: getting_started: Document building with west

Document building the samples with west (for now only in the Getting
Started guide).
Also switch to `reel_board` by default since `arduino_101` is no longer
well supported.

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

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