DOC: link to background info on fsaverage (mne-tools#13249)

Author: sappelhoff

doc/documentation/datasets.rst

@@ -398,7 +398,8 @@ fsaverage
 :func:`mne.datasets.fetch_fsaverage`
 
 For convenience, we provide a function to separately download and extract the
-(or update an existing) fsaverage subject.
+(or update an existing) fsaverage subject. See also the
+:ref:`background information on fsaverage <fsaverage_background>`.
 
 .. topic:: Examples
 

mne/datasets/_fsaverage/base.py

@@ -12,7 +12,7 @@
 
 @verbose
 def fetch_fsaverage(subjects_dir=None, *, verbose=None):
-    """Fetch and update fsaverage.
+    """Fetch and update :ref:`fsaverage <fsaverage_background>`.
 
     Parameters
     ----------

tutorials/forward/10_background_freesurfer.py

@@ -118,6 +118,8 @@
 # MNE-Python and FreeSurfer are integrated.
 #
 #
+# .. _fsaverage_background:
+#
 # 'fsaverage'
 # ===========
 #

tutorials/forward/35_eeg_no_mri.py

@@ -6,7 +6,7 @@
 ========================================
 
 This tutorial explains how to compute the forward operator from EEG data
-using the standard template MRI subject ``fsaverage``.
+using the standard template MRI subject :ref:`fsaverage <fsaverage_background>`.
 
 .. caution:: Source reconstruction without an individual T1 MRI from the
              subject will be less accurate. Do not over interpret activity