Allow dashboard sidebars to be collapsed by default, and other improvements

[pensivedog]

Description

Following Q&A #11144, this feature request feels like basic functionality that is missing.

On this note, I'd also like to suggest a general overhaul to the sidebar styling and transition effects, which seem quite clunky and not in keeping with the clean and professional appearance other Quarto features have right out of the box. Try toggling the open-closed state on this example, for reference:

https://mine-cetinkaya-rundel.github.io/ld-dashboard/

Specifically:

1. Bounciness is odd and a bit jerky
2. Toggle button (left arrow) overlays the vertical scroll control (up arrow) when sidebar is open and content requires vertical scrolling
3. Background color of sidebar remains visible as a narrow left strip and disappears last

Hopefully someone can bring this up to speed with other Quarto features!