Fix tutorial generation warnings

Fix some warnings when generating tutorials using `sphinx-build`

[new output (117 lines)](https://gist.github.com/VictorLamoine/b96bd9bcae1e8f6eb3bf)
[old output (146 lines)](https://gist.github.com/VictorLamoine/b96bd9bcae1e8f6eb3bf#file-old-output)
Makes sense. But maybe remove language specifier completely? it's not _cpp_ either.