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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[ArmandPhilippot]

Perhaps we can add a little more information based on your changeset:

Suggested change

	The hostnames that Astro is allowed to respond to. Refer to the [vite documentation](https://vite.dev/config/server-options.html#server-allowedhosts) for more information.
	Specifies the hostnames that Astro is allowed to respond to in `dev` or `preview` modes. Can be passed a comma-separated list of hostnames to allow or `true` to allow any hostname.
	Refer to the [Vite documentation](https://vite.dev/config/server-options.html#server-allowedhosts) for more information.

I also specified with which modes the flag works to prevent confusion if someone try to use this with the build command.