Doc: Conditionally building the jupyter extension

[LecrisUT]

I am not sure if this belongs to this project or another, but is it possible to conditionally run this builder hook? I.e. have a project that is primarily python based, but include python extensions. This issue came up in the request of this issue. I see in the documentation that you can add the dependencies to it, but would that only be pulled if the jupyter build is enabled?

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