DOC: Revise generation of examples to work in RTD

[oesteban]

Amends #3131.

[oesteban]

Hopefully, the last PR in the series following #3124, to make sure documentation builds on RTD.

[codecov]

Codecov Report

Merging #3132 into master will decrease coverage by 0.84%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3132      +/-   ##
==========================================
- Coverage    64.8%   63.95%   -0.85%     
==========================================
  Files         299      297       -2     
  Lines       39489    39408      -81     
  Branches     5220     5210      -10     
==========================================
- Hits        25589    25202     -387     
- Misses      12844    13162     +318     
+ Partials     1056     1044      -12

Flag	Coverage Δ
#unittests	63.95% <ø> (-0.85%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/nilearn.py	38.98% <0%> (-57.63%)	⬇️
nipype/utils/spm_docs.py	20% <0%> (-48%)	⬇️
nipype/interfaces/freesurfer/base.py	45.68% <0%> (-30.18%)	⬇️
nipype/utils/logger.py	58.46% <0%> (-27.7%)	⬇️
nipype/testing/fixtures.py	76.38% <0%> (-22.23%)	⬇️
nipype/interfaces/matlab.py	63.91% <0%> (-12.38%)	⬇️
nipype/interfaces/dcm2nii.py	48.82% <0%> (-9.86%)	⬇️
nipype/interfaces/spm/base.py	57.66% <0%> (-9.67%)	⬇️
nipype/pkg_info.py	75% <0%> (-6.25%)	⬇️
nipype/interfaces/io.py	50.07% <0%> (-6.12%)	⬇️
... and 12 more

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