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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Latest commit: cca1997

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