Open the help in the down area (pager)

[jtpio]

Fixes https://github.com/jupyter/notebook/issues/6692 Fixes https://github.com/jupyter/notebook/issues/1135

To offer a behavior similar to the classic notebook:

https://github.com/user-attachments/assets/4a47c0d4-3a67-4348-90ca-53c1ffd63e23

The idea would be to have this behavior by default in Notebook 7.5, with a setting to disable it and have the pager data be displayed in the cell output like in JupyterLab:

https://github.com/user-attachments/assets/344b8d56-0c30-4c74-aaba-592355288da7

Example with xeus-cpp showing a text/html bundle:

https://github.com/user-attachments/assets/632ed3a7-580d-4d76-9cf9-f9b8f7322c6e

• [x] Open the pager in the down area
• [x] Setting to disable this behavior and default to the JupyterLab behavior
• [x] If disabled, allow opening the "Contextual Help" and click around like in JupyterLab
• [x] UI test
• [x] Update snapshots
• [x] Docs

[github-actions[bot]]

:point_left: Launch a Binder on branch jtpio/notebook/down-pager

[jtpio]

So, I think this PR should be ready for a first round of reviews.

As a quick summary, it brings back the behavior of the pager just like the classic notebook. This has been missing since the early Jupyter Notebook 7 releases (released a bit more than two years ago), and there's been some feedback that this was still missing.

I've also added some UI tests to cover some cases, and also added a new page in the docs so that folks can opt out of this new behavior and return to the JupyterLab behavior if they want to.

If folks would like to try it, that would be great, let me know what you think. I'm also happy to iterate on a different approach if you think the current one could be improved.

[jtpio]

Thanks @andrii-i for the review 👍

Putting this back as a draft to revisit the handling of the kernel message. I will get back to it later, but if someone would like to pick it up, please feel free!