Docs: Document choose light or dark theme for sidebars

If you're creating a sidebar with a dark background in an otherwise light theme, sidebar() will at least choose the contrasting light color for the foreground color. But there will still be many elements that may have inappropriate styles.

One potentially easy way to fix this is to use the dark color mode styles for everything, locally scoped to the sidebar:

layout_sidebar(
  sidebar = sidebar(
    title = "Sidebar",
    p("Light text on a dark background."),
    actionButton("click_me", "Click Me"),
    bg = "purple",
    "data-bs-theme" = "dark"
  ),
  h3("Main content"),
  p("Dark text on a light background.")
)

This might not be a complete fix, depending on the dark color mode styles for your current theme, but it can often get you very close to where you want to be.

Start from the sidebar() and layout_sidebar() documentation and review the shown data-bs-theme = "dark" example against the existing theme guidance. Done means the docs explain how to scope dark color-mode styles to a sidebar in an otherwise light layout, including the limitation that results depend on the current theme.