docs: add allowedHosts to PreviewServerParams reference

[ViVaLaDaniel]

This PR adds the missing \�llowedHosts\ property to the \PreviewServerParams\ reference in the Adapter API documentation. This property was added to Astro core to support host restriction in adapter preview servers (see withastro/astro#16290).

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[ArmandPhilippot]

Thank you, and welcome. I can see the implementation PR has been approved, docs LGTM too! 🎉

But, please keep our PR template the next time.

[yanthomasdev]

LGTM! Thanks!