DOC: use footcite in mne/datasets/sleep_physionet/age.py (#9105)
DOC: use footcite in mne/datasets/sleep_physionet/age.py (#9105)

* convert references in mne/datasets/sleep_physionet/age.py to use footcite / footbibliography

* convert references in mne/datasets/sleep_physionet/age.py to use footcite / footbibliography

* convert references in mne/datasets/sleep_physionet/age.py to use footcite / footbibliography

* trailing whitespace errors fixed

* Update mne/datasets/sleep_physionet/age.py

Co-authored-by: Daniel McCloy <2591e5f46f28d303f9dc027d475a5c60d8dea17a@mccloy.info>

* add blank line between summary and description

* Reverting mistake

* Add blank line between summary and description

Co-authored-by: Daniel McCloy <2591e5f46f28d303f9dc027d475a5c60d8dea17a@mccloy.info>
mne-tools/mne-python,mne-tools/mne-python,larsoner/mne-python,olafhauk/mne-python,Teekuningas/mne-python,Eric89GXL/mne-python,wmvanvliet/mne-python,Teekuningas/mne-python,larsoner/mne-python,kambysese/mne-python,pravsripad/mne-python,kambysese/mne-python,rkmaddox/mne-python,larsoner/mne-python,kingjr/mne-python,kingjr/mne-python,pravsripad/mne-python,pravsripad/mne-python,drammock/mne-python,bloyl/mne-python,wmvanvliet/mne-python,Eric89GXL/mne-python,drammock/mne-python,kingjr/mne-python,Teekuningas/mne-python,wmvanvliet/mne-python,olafhauk/mne-python,drammock/mne-python,olafhauk/mne-python,bloyl/mne-python,mne-tools/mne-python,rkmaddox/mne-python