[Docs] Codesandbox environments aren't quite correct for legacy theme

[cchaos]

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I know the legacy theme is on it's way out, so solving this issue is more dependent on how much effort it would require.

But it looks like the hook useEuiTheme() will always return the Amsterdam theme even if no EuiProvider is wrapping the applicaiton. The creates a contradiction when using the codesandbox links originating while the legacy theme is selected and using that hook.

Example:

That screenshot originates from the updated example in this PR: #5623

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