Add build.assetsPrefix to configuration reference

[rururux]

Description (required)

Both the deprecation comments in Astro's source code and the "Deprecated: import.meta.env.ASSETS_PREFIX" section on the "Upgrade to Astro v6" page recommend importing build.assetsPrefix from astro:config/server as a replacement for import.meta.env.ASSETS_PREFIX.
However, build.assetsPrefix was missing from the "Imports from astro:config/server" reference.

This PR adds build.assetsPrefix to the "Imports from astro:config/server" list.

Related issues & labels (optional)

withastro/astro#16604
withastro/astro#16605

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[ArmandPhilippot]

Thank you, LGTM! (even if this is still a draft) We can merge this when withastro/astro#16653 is merged.

[rururux]

Thank you for the approval!
I had kept this PR as a draft because I was waiting for the implementation PR to be merged, and now that both that and the release PR have been merged, this PR is ready to merge as well!

[ArmandPhilippot]

This is not a big deal for this one as this is a small fix! But for your future docs contributions: it's better if you undraft the PR when it's ready. We have labels to be sure docs are not merged before the release.

A PR in draft means this is not yet ready for review so we usually don't check them. This could block a release: if the associated docs PRs do not have an approval, we may delay merging the [ci] release PR. We merge docs at the same time, not later. Users might check what is written in docs when they see a new release!

And feel free to merge your PRs once they are approved, you now have the right to do so! 😉

[rururux]

Thanks for merging, and for the tips!
I'll keep that in mind for future PRs. 🙏