infra: add a step to validate the content of notebooks

[maximlt]

Precisely, to warn/fail when a notebook contains code outputs when it shouldn't, or doesn't contain any when it should (skip_notebooks_evaluation: True). The goal of this additional validation step is to make it less likely to commit a notebook with code outputs, which is a common gotcha.

cc @jtao1 @Azaya89

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).

[Azaya89]

Is there a doit command we should run to activate this step?

[maximlt]

It should run by default when you call doit validate:<project>. It will also run on the CI.