Add support for custom kernel provision

[agoose77]

Context

jupyter-book should ideally be an application that one runs with pipx. However, right now it assumes that the user kernels are installed into the same environment as the documentation engine. It would be nice if we could separate the two.

Proposal

Introduce a mechanism for finding external kernels that should be used for notebook execution.

Tasks and updates

No response

[flying-sheep]

Oof yeah real nice. Actually my current project depends on that: https://github.com/flying-sheep/session-info2

The absence of this means that have to separate notebook execution from document building, which is a bit complex for such a small project.