docs: api: uart: Restructure and provide introduction
docs: api: uart: Restructure and provide introduction

The document is now structured into the three different APIs. In
addition to that, a section for relevant Kconfig options is added.

Signed-off-by: Martin Jäger <54669547a225ff20cba8b75a4adca540eef25858@libre.solar>

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