Add example of reading script from stdin using echo (#6567)

As a non-shell-wizard, I was unfamiliar with the `EOF` syntax used in
the existing example (just above the one I added). I thought including
an example where the output of `echo` is piped to `uv run` might be more
accessible. As a bonus, it should work across more shells: the `EOF`
example doesn't work in fish because fish [doesn't support
heredocs](https://fishshell.com/docs/current/fish_for_bash_users.html#heredocs),
while the `echo` example does.

Feel free to ignore if unwanted.

docs/guides/scripts.md

@@ -57,6 +57,12 @@ hello world!
 
 Additionally, your script can be read directly from stdin:
 
+```console
+$ echo 'print("hello world!")' | uv run -
+```
+
+Or, if your shell supports [here-documents](https://en.wikipedia.org/wiki/Here_document):
+
 ```bash
 uv run - <<EOF
 print("hello world!")