fix(docs): resolve accessibility report issues

[dancormier]

Summary

• Fixes a few Stacks docs accessibility report issues found at https://accessibility.stackoverflow.net/products/stacks/automated
• Adds accessible names/roles for affected examples and improves docs-only contrast on some text and links.
• Wraps the toggle button demo in the standard example container (not strictly accessibility but just cleanup)

Testing

• Load up the deploy preview
• Ensure N/A in class tables on our component docs pages (such as Avatars) meet our contrast minimums
• Ensure page description (below title) meets our contrast minimums
• Ensure links within docs page text content (such as the Popper.js link in the Popover docs) meet our contrast minimums
• Ensure each scrollable overflow atomic class example can be tabbed to and scrolled with arrow keys as appropriate (not all will be tabbable/scrollable)

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[asblanco]

Generated markdown prose links still seem to inherit the lower-contrast paragraph color. addDocClasses gives paragraphs docs-copy and anchors docs-link, and the existing .docs-copy .docs-link { color: inherit; } rule is more specific than this new .docs-link rule. That means links like the Popper.js example stay on the inherited paragraph color instead of using --theme-link-color / --black-600.

Could we either remove/update the earlier inherited-color rule, or include the prose selector here so it wins?

Suggested change

	color: var(--theme-link-color, var(--black-600));
	.docs-copy .docs-link,
	.docs-link {
	color: var(--theme-link-color, var(--black-600));
	font-weight: 500;
	}