UI: Preferred color scheme awareness

[mathiasrando]

Issue: #6088

What I did

• Add awareness for preferred color scheme and set the default theme accordingly
• Update the theming documentation to mention the awareness

How to test

1. Use a browser that supports prefers-color-scheme
2. Set your preferred color scheme
3. Open a Storybook without a provided theme
   3a) Clear your localStorage and refresh the page if it's not already clean
4. Check that it shows the dark/light theme matching your preference
5. Change your preferred color scheme
6. Refresh the page
7. Check that it shows the dark/light theme matching your preference

If anyone has an idea for updating the tests for create and convert help would be greatly appreciated. Unfortunately I couldn't change the mock of watchMedia during the test and therefore updating the existing tests to account for the preferred color scheme awareness wasn't possible. I decided to leave the existing tests as they still pass since the light theme is still handled as the default.

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[shilman]

This looks fantastic. Nice tests!