Add "importing secondary color palettes" to customize docs

[nothingrandom]

Summary

This pull request adds a new section to the customize.mdx documentation page, explaining how to import secondary color palettes in Park UI. The change provides an example of using a green color palette for success states.

Justification

I think this is a beneficial change due to to the topics found here;

• Allow all themes to be available #170
• [feature request] panda's dynamic themes #278
• [Bug]: Accent color doesnt work with custom extended colors #453
• Missing additionalColors property in createPreset function #485
• CreatePreset additionalColors Support #508

It is, notably, #485, which is where this solution comes from. But with the number of issues having it officially in the documentation would be great.

Alternatives

Of course, if we could tap into the additionalColors as resolved by this PR; #509 that would be far more ideal.

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