doc: dts: Explain better how devicetree and Kconfig fit together
doc: dts: Explain better how devicetree and Kconfig fit together

It's cryptic that some identifiers for devicetree-related stuff start
with DT_*, while others start with CONFIG_*. Explain what's going on,
and link to the Kconfig preprocessor documentation.

Also remove an early mention of bindings that might be confusing.
Bindings are much more about checking devicetree conformance than about
controlling output, though they do some of that too.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

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