[MAINT, MRG] remove voxels from example

[alexrockhill]

bids-standard/bids-specification#1031 specifies that template data should be in scanner RAS so we should remove the voxel example and clarify that surface RAS == scanner RAS for fsaverage.

Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

Merging #988 (dffd887) into main (d717ea7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #988   +/-   ##
=======================================
  Coverage   95.07%   95.07%           
=======================================
  Files          25       25           
  Lines        3713     3713           
=======================================
  Hits         3530     3530           
  Misses        183      183           

Impacted Files	Coverage Δ
mne_bids/dig.py	98.43% <ø> (ø)

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[agramfort]

thx @alexrockhill !