feat: document external redirects

[ematipico]

Description (required)

This PR documents the upcoming new feature, where external redirects are supported.

For Astro version: 5.2.0. See astro PR #12979.

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Warnings reference

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[sarah11918]

Thanks @ematipico ! I made a commit here because it was otherwise awkward to suggest here in GitHub, but I don't think this needs to be a standalone section (where it's easy to read all the stuff saying "redirects are to another route...." for lots of paragraphs and then see, "Oh, or you can do this."

See what you think of this edit, and can we work with something like this?

[sarah11918]

Approving for the next minor release! 🎉