Add DOI for Heybrock et al. 2023 paper in docstrings/comments

Places where the reference needs to be updated:
- `src/ess/reflectometry/corrections.py`
- `src/ess/sans/normalization.py`
- `src/ess/sans/i_of_q.py`
- `src/ess/uncertainty.py`
- `docs/instruments/loki/sans2d_to_I_of_Q.ipynb`
- `docs/instruments/external/powgen/powgen_reduction.ipynb`
