Make testpypi index explicit in example snippet (#12148)

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## Summary

Update this example snippet adding test.pypi.org as a publishing index
to mark the index with `explicit = true`. This will help prevent users
from unexpected behavior if no other indices are defined and users don't
select a different index selection algorithm (with `--index-strategy`).
When `test.pypi.org` is the selected index for package management,
packages resolve to odd versions like 0.0.1 and `uv` spits out lots of
errors.

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## Test Plan

N/A, documentation only change
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docs/guides/package.md

@@ -82,6 +82,7 @@ If you're using a custom index through `[[tool.uv.index]]`, add `publish-url` an
 name = "testpypi"
 url = "https://test.pypi.org/simple/"
 publish-url = "https://test.pypi.org/legacy/"
+explicit = true
 ```
 
 !!! note