docs: color scheme switcher

[reidbarber]

Adds a color scheme switcher to the header for React Spectrum pages.

• Switches between system preference and the other
• Doesn't apply to React Aria currently (it will be tricky to get it to fully work with the examples)

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Test the color switcher on S2 docs pages (including the home page).

🧢 Your Project:

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[snowystinger]

Nice! Putting the whole app into the colorscheme that isn't your OS colorscheme causes the search overlay to be wrong

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[snowystinger]

Noticed that the bottom of the page https://d5iwopk28bdhl.cloudfront.net/pr/fde4d39fe83e28091754a70459dbaa4c7acdc8b2/ there is no settings, also the /'s are doubled up for some reason. Looks like it's already this way on main, so pre-existing

Also, some things are meeting contrast, like the Explore Components button. It's like some things switched, but not others if my computer is in dark mode but my settings are in light mode :-/

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[snowystinger]

It still won't put the search dialog into the correct color scheme, is that alright?

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the S2 docs
• View the React Aria docs

[snowystinger]

a little unfortunate that it flickers if your computer is in dark mode on a page load

[snowystinger]

could we visibility hidden the button until we know the current color scheme?