Create a reference React Doc

[avivkeller]

Before writing the code to convert our MDX into React, we should create a reference example. It doesn't need to live in this repo, but it should match the design and use @node-core/ui-components, so we can refer to it later when building the JSX generator.

See https://github.com/avivkeller/nodejs-docs-redesign-reference for an (in-progress) reference doc.

[avivkeller]

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The image above shows how the plain markdown will be rendered. The goal of the reference document is to outline the transformations we need to apply to the markdown in #6 so that it matches the appearance shown in Figma.