doc: getting_started: Add WSL instructions
doc: getting_started: Add WSL instructions

Having tried and tested building Zephyr using the standard SDK on
Windows 10 using the new WSL (Windows Subsystem for Linux), add the
documentation so that others can benefit from the functionality.

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

holtmann/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,runchip/zephyr-cc3220,rsalveti/zephyr,nashif/zephyr,rsalveti/zephyr,kraj/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,ldts/zephyr,fractalclone/zephyr-riscv,Vudentz/zephyr,ldts/zephyr,mbolivar/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,kraj/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,Vudentz/zephyr,ldts/zephyr,runchip/zephyr-cc3220,aceofall/zephyr-iotos,explora26/zephyr,finikorg/zephyr,fbsder/zephyr,galak/zephyr,mbolivar/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,sharronliu/zephyr,runchip/zephyr-cc3220,Vudentz/zephyr,kraj/zephyr,nashif/zephyr,fractalclone/zephyr-riscv,finikorg/zephyr,finikorg/zephyr,punitvara/zephyr,fbsder/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,explora26/zephyr,galak/zephyr,holtmann/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,holtmann/zephyr,punitvara/zephyr,fbsder/zephyr,aceofall/zephyr-iotos,sharronliu/zephyr,kraj/zephyr,mbolivar/zephyr,fractalclone/zephyr-riscv,holtmann/zephyr,fractalclone/zephyr-riscv,sharronliu/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,rsalveti/zephyr,fractalclone/zephyr-riscv,Vudentz/zephyr,punitvara/zephyr,zephyriot/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,ldts/zephyr,nashif/zephyr,rsalveti/zephyr,explora26/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,zephyriot/zephyr,runchip/zephyr-cc3220,rsalveti/zephyr,fbsder/zephyr,sharronliu/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,finikorg/zephyr,holtmann/zephyr,Vudentz/zephyr,fbsder/zephyr,sharronliu/zephyr