Address website feedback

[srmagura]

This addresses the four pieces of feedback that were left on #2805:

• Make the remarkFixLinks plugin rewrite GitHub URLs, e.g.: https://github.com/emotion-js/emotion/tree/main/packages/babel-plugin ➡ /docs/@emotion/babel-plugin
• Revert the tables I converted to HTML back to GFM tables, and add the remarkGfm plugin.
• Slightly tweak the DocSearch styles to hopefully better match the rest of the site. Now it's pink rather than blue when you hover over it.
• Add back .nvmrc with Node 16.

Also:

• Remove style override I added that made non-live code blocks have large font.

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Latest commit: dd8eff3

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