DOC: Add example notebooks to sphinx documentation

[mgxd]

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[codecov-commenter]

Codecov Report

Merging #126 (2e911fd) into master (02b5426) will increase coverage by 0.25%.
The diff coverage is 99.15%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   98.63%   98.89%   +0.25%     
==========================================
  Files          11       12       +1     
  Lines         951     1084     +133     
  Branches      129      138       +9     
==========================================
+ Hits          938     1072     +134     
+ Misses          7        6       -1     
  Partials        6        6              

Flag	Coverage Δ
travis	98.89% <99.15%> (?)
unittests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nitransforms/io/fsl.py	94.93% <85.71%> (+0.19%)	⬆️
nitransforms/io/afni.py	98.75% <94.44%> (+0.04%)	⬆️
nitransforms/__init__.py	100.00% <100.00%> (ø)
nitransforms/base.py	98.64% <100.00%> (-0.78%)	⬇️
nitransforms/cli.py	100.00% <100.00%> (ø)
nitransforms/io/__init__.py	100.00% <100.00%> (ø)
nitransforms/io/base.py	100.00% <100.00%> (ø)
nitransforms/io/itk.py	100.00% <100.00%> (ø)
nitransforms/io/lta.py	99.34% <100.00%> (+0.01%)	⬆️
nitransforms/linear.py	97.60% <100.00%> (+1.52%)	⬆️
... and 10 more

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[mgxd]

Rendered: https://nitransforms--126.org.readthedocs.build/en/126/