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 <dan@mccloy.info>

* add blank line between summary and description

* Reverting mistake

* Add blank line between summary and description

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

mne/datasets/sleep_physionet/age.py

@@ -22,7 +22,8 @@ def fetch_data(subjects, recording=(1, 2), path=None, force_update=False,
     """Get paths to local copies of PhysioNet Polysomnography dataset files.
 
     This will fetch data from the publicly available subjects from PhysioNet's
-    study of age effects on sleep in healthy subjects [1]_[2]_. This
+    study of age effects on sleep in healthy subjects
+    :footcite:`MourtazaevEtAl1995,GoldbergerEtAl2000`. This
     corresponds to a subset of 153 recordings from 37 males and 41 females that
     were 25-101 years old at the time of the recordings. There are two night
     recordings per subject except for subjects 13, 36 and 52 which have one
@@ -81,14 +82,7 @@ def fetch_data(subjects, recording=(1, 2), path=None, force_update=False,
 
     References
     ----------
-    .. [1] MS Mourtazaev, B Kemp, AH Zwinderman, HAC Kamphuisen. Age and gender
-           affect different characteristics of slow waves in the sleep EEG.
-           Sleep 18(7):557–564 (1995).
-    .. [2] Goldberger AL, Amaral LAN, Glass L, Hausdorff JM, Ivanov PCh,
-           Mark RG, Mietus JE, Moody GB, Peng C-K, Stanley HE. (2000)
-           PhysioBank, PhysioToolkit, and PhysioNet: Components of a New
-           Research Resource for Complex Physiologic Signals.
-           Circulation 101(23):e215-e220
+    .. footbibliography::
     """  # noqa: E501
     records = np.loadtxt(AGE_SLEEP_RECORDS,
                          skiprows=1,