doc: Fix broken links in getting_started
doc: Fix broken links in getting_started

Fix broken links in getting_started. We now use "Implicit Hyperlink
Targets" instead of :ref:. I don't know why :ref: wasn't working.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

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