* Modify Doxygen grouping of `drivers` Public/Internal APIs
* Correct classification of `mbed_events.h`
* Amend name of Doxygen group containing Device Key API
* Classify `CallChain.h` as public API and relocate file
* Remove Doxygen group from `equeue_platform.h` as it has no Doxygen compliant documentation
* Move USB target specific code back to `usb/device/targets`
* Change Doxygen groups structure, splitting first by Public/Internal
This commit also does the following:
* groups the documentation of related API
* moves `events/internal/equeue.h` to `events/equeue.h`
* merges `events/source/README.md` to `events/README.md`
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
Hugues Kamba