Remove usage of `NIFTy` in docs

nifty5/data_objects/numpy_do.py

nifty5/domains/gl_space.py

nifty5/domains/hp_space.py

nifty5/domains/lm_space.py

nifty5/domains/log_rg_space.py

nifty5/domains/power_space.py

nifty5/domains/rg_space.py

nifty5/domains/structured_domain.py

nifty5/field.py

nifty5/library/amplitude_operator.py

nifty5/operators/diagonal_operator.py

nifty5/operators/endomorphic_operator.py

nifty5/operators/scaling_operator.py