Fix nbshpinx rendering of tutorials

[fritzo]

I was having trouble generating the index and seeing lots of errors like

WARNING: toctree contains reference to document 'gp' that doesn't have a title: no link will be generated

Removing ".ipynb" from the source_suffix in tutorial/source/conf.py seemed to resolve the issue.

See these posts for similar issues

• https://stackoverflow.com/questions/14079655/cannot-get-toctree-in-sphinx-to-show-link
• https://stackoverflow.com/questions/55297443/including-notebook-with-nbsphinx-fails