feat(sidebar): add Colors category to Guides

[bsmth]

Summary

This PR adds a new section for the CSSRef sidebar which links to some guides on Colors

Problem

We do not surface Colors guides in CSS section as readily as we could

Solution

Highlight existing guides in the CSS Sidebar. Adds links to:

• Applying color to HTML elements using CSS
• Web Accessibility: Understanding Colors and Luminance
• Web Accessibility: Color Contrast

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Screenshots

Before

After

How did you test this change?

yarn && yarn dev

[caugner]

Changes LGTM, but my concern is that the two accessibility URLs have a different sidebar, and this might be confusing for users.

[bsmth]

two accessibility URLs have a different sidebar, and this might be confusing for users.

Yes, I think this is a good thing to watch out for, I was also considering this, but I think it's reasonable given we're surfacing important info under that topic. It would be great to watch closely and revert or fix if this is confusing for readers.