docs(color-modes): Add document about color value can be obj/arr/hex.

[PeterlitsZo]

For issue #2123

[vercel[bot]]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/systemui/theme-ui/FK8joXpeWT7hbBg3Q849eY8v7HKC
✅ Preview: https://theme-ui-git-fork-peterlitszo-update-doc-for-co-b1cb77-systemui.vercel.app

[codesandbox-ci[bot]]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 4426ea84f1cb373bb85ff8e1214307d33d1e8e5b:

Sandbox	Source
next-theme-ui-example	Configuration
gatsby-plugin-theme-ui-example	Configuration

[hasparus]

I think that part of the docs was more of "How does Theme UI color modes logic work?" instead of "How to define your color modes?". We might need to rewrite this whole page when I think about it.