feat: document new CLI flag

[ematipico]

Description (required)

It documents a new CLI flag called --allowed-hosts.

I am not very good at this part, I just referenced the Vite documentation.

For Astro version: 5.4.0. See astro PR #13278

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[sarah11918]

Fun fact: I just had to configure vite.server.allowedHosts yesterday while working on my site so that Gitpod could display the preview in the browser 😄

Suggesting we accept Armand's edit (mostly word for word) if that sounds good to you, @ematipico !

[sarah11918]

Looks great! (And, now I'll be able to remove a vite config in my own project... 😄 )