Disable Jinja for notebooks

[encukou]

It looks like jinja: false would be a good default for .ipynb sources.

[hroncok]

I agree. Let's try that and diff the results?

[encukou]

If you have time, go ahead :)
Notes:

• The naucse_render version is pinned in each repo, so we can do explicit upgrades.
• It would be good to try with the data courses, which use notebooks heavily.