Power management stat : add verbosity level for MBED_SLEEP_TRACING_ENABLED ...

Full verbosity is adding a console line for each lock/unlock API call

- stats can be enabled with json config
- default configuration is full verbosity and add a console line for each lock/unlock command
- for STM32 targets, verbosity is reduced by default

Platform: Separate internal APIs from public APIs ...

Also includes:
* rename `mbed_sleep_manager.c` to `mbed_power_mgmt.c` to match its
  header file
* create Doxygen groups for public and internal APIs
* use relative path to include header files where inconsistent
* update references to internal APIs throughout libraries
* update the copyright year for all modified files