Document the script mode

[evnchn]

Motivation

We can't talk about not using the script mode in #5297, if we did not document what that even is.

Implementation

Add a small section on what is script mode in the documentation.

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).

[falkoschindler]

Thanks, @evnchn!
But you're referring to the root function, which is also not explicitly documented. This is an open task on #5109.
@rodja Are you still planning to write a section about the root parameter? If so, we should coordinate our efforts.

[rodja]

Yes, root pages must also get better documentation. I took a first step with #5275. If that is through I want to update examples and also some of the demos where @ui.page is currently used. I think this can be an independent, and useful PR.