docs: many, many minor and significant improvements to docs

[dancormier]

This PR make changes to our docs, many of which have been a long time coming. I went page-by-page and tried to unify the look and markup.

I had a few general goals:

• Simpler markup that leverages loops
• Greater reliance on Stacks components and atomic classes
• Fewer busted looking examples

Notable changes

• Update the landing page with styles more inline with current Stacks; improve responsiveness
  • I'm not entirely satisfied with the look of the card links and may revisit
• New logo in the topbar (plus some refined styles)
• Update page layout to better leverage flexbox instead of relying on absolute positioning
• Replace many custom docs styles with Stacks classes
• Rely almost entirely on rem-based units
• Table of Contents now renders as a .s-navigation component
• Removed nearly every .flex--item class from the docs markup

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Latest commit: 68b8729

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