add new example for plotting on realistic surfaces

[sappelhoff]

to do:

• get it to run on readthedocs (headless)
  • may need use of custom commands
• add actual documentation not only the plots
• change order: first plot on sphere, then on head
• show how mne standard montage is actually not computed on a sphere

Setup a robust way to get fsaverage data, maybe through https://nightly.link/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.96%. Comparing base (1481572) to head (12ead6e).
Report is 25 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
+ Coverage   97.47%   97.96%   +0.49%     
==========================================
  Files           5        5              
  Lines         396      394       -2     
==========================================
  Hits          386      386              
+ Misses         10        8       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.