doc: add stdin to guide (#6531)
<!--
Thank you for contributing to uv! To help us out with reviewing, please
consider the following:
- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->
## 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 https://github.com/astral-sh/uv/pull/6481
```bash
$ uv run - <<EOF
import antigravity
EOF
```
## Test Plan
<!-- How was it tested? -->
https://github.com/astral-sh/uv/pull/6519#issuecomment-2307371063 new PR
