docs(examples): add theming example

[joshblack]

Add a theming example to our examples directory. This example is to help us test out changes to ThemeProvider as we move over to CSS Modules.

Changelog

New

• Add examples/theming example with Next.js App Router

Changed

Removed

Rollout strategy

• None; if selected, include a brief description as to why

This is a local example that will not impact the public API of a Primer package

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Latest commit: a9e1964

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[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	97.47 KB (0%)
packages/react/dist/browser.umd.js	97.8 KB (0%)

[jonrohan]

Awesome 👍🏻

[jonrohan]

Should we note that <ThemeProvider here is temporary?

[joshblack]

No preference on my end, happy to add it if it would be helpful 👍

[jonrohan]

Like the idea that this could one day be a helper function

Suggested change

	data-color-mode={colorMode}
	data-light-theme="light"
	data-dark-theme="dark"
	{...colorThemeAttributes(colorMode)}