refs https://github.com/TryGhost/Product/issues/4105
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### <samp>🤖 Generated by Copilot at 2edba98</samp>
This pull request introduces a new monorepo package called
`admin-x-design`, which contains components, design guidelines and
documentation for building apps in Ghost Admin. It also moves some
existing components and files from the deprecated `admin-x-settings`
package to the new `admin-x-design` package, and updates some styles and
rules to use TailwindCSS. The purpose of these changes is to improve the
consistency, maintainability and usability of the Ghost Admin UI.
