Remove nesting color modes docs section

[mdo]

Officially drops full support for nesting light mode components within dark mode components, given the issues around needing to duplicate selectors. For those following along, or finding this later, this is essentially what we'd need to do to properly support nesting light in dark:

.carousel,
.carousel[data-bs-theme="light"],
[data-bs-theme="light"] .carousel {
 //...
}

And this would need to be applied to basically every single selector. For the time being, this just isn't practical, and most likely not even that useful.

Closes #38065.