content: playground example for condensed header usage with virtual scroll

### URL

https://ionicframework.com/docs/api/header#usage-with-virtual-scroll

### Issue Description

When triaging #25463, it was identified that the documentation around virtual scroll usage is confusing with the header. The fade implementation does not require to be rendered inside the virtual scroll container, but the condensed header does.

We should render a playground example for the condensed header to help developers navigate this difference.

Start at the header documentation URL and review the existing usage guidance around virtual scroll. Add a playground example for condensed header usage that makes clear it must be rendered inside the virtual scroll container, while the fade implementation does not; the documentation should resolve this distinction for developers.

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