[AX] Color scheme toggle keyboard navigation should follow radio buttons standards

[marinaaisa]

Bug/issue #94221573, if applicable:

Summary

Color scheme toggle keyboard navigation should follow radio buttons standards: https://www.w3.org/WAI/ARIA/apg/example-index/radio/radio.html

radio.mov

Dependencies

NA

Testing

Steps:

1. Run any doccarchive
2. Go to footer
3. Tab and assert that you focus the radio buttons for the Color scheme toggle
4. Assert that by pressing the arrow keys you can select different color schemes

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran npm test, and it succeeded
• Updated documentation if necessary

[dobromir-hristov]

I see you did some general cleanup along with the change. I think that is fine.

I had the focus-within mixin pending for some time now in, good we are adding it - https://github.com/apple/swift-docc-render/pull/348/files#diff-96c76e12b6abbe6ec6df39ce6b01d2fb4b418d3569baf7dd041536fff6dd2be8

[marinaaisa]

@swift-ci test