docs: Add information about dynamic static split server islands

[Jacob-Roberts]

Description (required)

This PR is the documentation follow-on to this PR which adds the ability to serve the server islands from a separate domain.

For Astro version: 5.1. See astro PR 12560.

First-time contributor to Astro Docs?

jake_roberts

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.