doc: arm: Update stm32_min_dev doc
doc: arm: Update stm32_min_dev doc

Updates the documentation to show PC13 instead
of PB12 as deafult LED Pin. Also adds note
about the default LED based on version of board.
Includes the change that needs to be made to
the dts file for changing default LED pin

Signed-off-by: Suryansh Sharma <d86b4781cfda87b0c94023af7aaa79d9d4e12a60@evilscientist.cc>

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