doc: add stdin to guide (#6531)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->
Document in guide stdin usage

alllll the easter eggs can do as well, but declined to keep consistent
with the other examples 😆

Additions to #6481

```bash
$ uv run - <<EOF               
import antigravity
EOF
```

## Test Plan

<!-- How was it tested? -->


#6519 (comment) new PR

docs/guides/scripts.md

@@ -55,6 +55,14 @@ $ uv run example.py hello world!
 hello world!
 ```
 
+Additionally, your script can be read directly from stdin:
+
+```bash
+uv run - <<EOF
+print("hello world!")
+EOF
+```
+
 Note that if you use `uv run` in a _project_, i.e. a directory with a `pyproject.toml`, it will
 install the current project before running the script. If your script does not depend on the
 project, use the `--no-project` flag to skip this: