Clean up Shiny docs (R and Python)

Shiny for Python docs currently live under Guide > Dashboards > Interactivity > Shiny for Python

Shiny for R docs mostly live under Guide > Interactivity > Shiny (i.e. not under Dashboards, although some pages are included in both locations Guide > Dashboards, and Guide > Interactivity).

This currently leads to confusion:

• Navigating to Guide > Interactivity > Shiny > Running Documents, drops you into the Dashboard docs
• Shiny for Python isn't mentioned in Guide > Interactivity

Some questions to consider:

• How do "Guide > Interactivity" and "Guide > Dashboards > Interactivity" differ? Do we need both? What content can be shared? How should they be linked? What distinctions do we want to outline for users?

• Do we still need Component Layout https://quarto.org/docs/interactive/layout.html ? I think this page should focus on the layout of components in longer form articles, and link to dashboards for full page layouts.

• Where do we talk about the difference between a Quarto dashboard with Shiny Python, and a Shiny Python app (ditto for R): no server() function, no ui layout functions e.g. ui.page_fluid() etc, no call to App(), no @output decorator (python), adding #| context: server (R)? Some mentions in various places, but it would be nice to have some side by side comparisons of code.