doc: dts: create zephyr chosen nodes section
doc: dts: create zephyr chosen nodes section

Split the API docs on chosen nodes from the reference table of
zephyr-specific chosen nodes. They are related but logically different
and I often want to link to just the table, while skipping the API.

Signed-off-by: Martí Bolívar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr