NUCLEO-L031K6 low power on MBED OS 2

@AnnaBridge that board is not supported in MbedOS6+
@ruck3rs the last where that target was supported seems to be MbedOS 5.15 also under bare metal profile.

The documentation was usually created from code directly I think, so maybe you can found something in the source - mbed-dev or mbed

BR, Jan