KEY-234: Port showcase to Docs site

[simonswiss]

BIG SCARY PR!

There are many files changed. Here's the gist of it:

• Ports the showcase website (currently a standalone Astro site) into the docs Next.js site
• Implements the new Figma design
• Reworks the colors throughout the Keystatic docs with the Keystart UI Tokens (overwrite default colors and make sure the UI doesn't use any default color)
• Flows element of the brand refresh (colors, font weight, badges etc) across the docs pages beyond just the showcase.
• Change the Tailwind config to a TS file
• Use Keystatic Cloud for the showcase images

Not ideal to review the PR, but a bunch of files only changed because I setup the prettier-plugin-tailwindcss properly on the docs project — which sorts Tailwind classes consistently.

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Latest commit: 9ff5137

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Name	Status	Preview	Comments	Updated (UTC)
keystar-ui	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 5, 2023 11:31am
keystatic	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 5, 2023 11:31am
keystatic-site	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 5, 2023 11:31am