NiusIMU 0.2.1

Prepare the IMU library for Arduino Library Manager registration.\n\nChanges:\n- Rename the Library Manager display name to NiusIMU to comply with the third-party library naming rule.\n- Add the matching primary include header NiusIMU.h while keeping ArduinoNRF_IMU.h available.\n- Update README/docs to explain the Library Manager name and repository history.\n- Verified with arduino-lint strict Library Manager submission checks and all examples compile for arduinonrf:nrf52:promicro_nrf52840.