Make the sidebar scrollable above 768px

[rik]

I tested those changes in the devtools but I did not build the docs to check (it would be nice to have instructions on how to contribute)

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[josepmartins]

Thank you @rik, looks good to me! ✨

We have a contributing doc for contributions. Other than that, the doc site is a Jekyll project, so to run it locally you'll need to cd docs & bundle exec jekyll serve