Update scripts docs with package indexes (#11443)

## Summary

The [current scripts docs
page](https://docs.astral.sh/uv/guides/scripts/) doesn't include detail
on how to use a custom package index when setting up a script. I believe
this might be because it didn't use to work (see #6688 ) but it now does
(thanks for that, by the way! 😄)

Given it's a useful feature, I suggest adding a quick example to the
scripts page, with the details of authentication, etc. left to the main
`indexes.md` doc.

I'd also suggests that this closes #6688, though it doesn't actually add
that feature - that appears to have already been done :)

## Test Plan

No testing is needed, I think!

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

docs/guides/scripts.md

@@ -217,6 +217,25 @@ print(Point)
 is not installed — see the documentation on [Python versions](../concepts/python-versions.md) for
 more details.
 
+## Using alternative package indexes
+
+If you wish to use an alternative [package index](../configuration/indexes.md) to resolve
+dependencies, you can provide the index with the `--index` option:
+
+```console
+$ uv add --index "https://example.com/simple" --script example.py 'requests<3' 'rich'
+```
+
+This will include the package data in the inline metadata:
+
+```python
+# [[tool.uv.index]]
+# url = "https://example.com/simple"
+```
+
+If you require authentication to access the package index, then please refer to the
+[package index](../configuration/indexes.md) documentation.
+
 ## Locking dependencies
 
 uv supports locking dependencies for PEP 723 scripts using the `uv.lock` file format. Unlike with