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.
