[MCCS-163] Update adminMode state machine and write docs for it

docs/source/State_Machine.rst

docs/source/images/AdminModeStateMachine.png

docs/source/images/DeviceStateMachine.png

docs/source/images/ObservationStateMachine_adr8.png

docs/source/images/ObservationStateMachine_autogenerated.png

docs/source/images/OperationalStateStateMachine_coupled.png

.../source/images/OperationalStateStateMachine_decoupled.png

docs/source/images/device_state_diagram.png

src/ska/base/base_device.py

src/ska/base/state_machine.py

tests/data/admin_mode_state_machine.json

tests/data/device_state_machine.json

tests/data/subarray_state_machine.json