[rustdoc] Make the buttons remain when code example is clicked
Follow-up of https://github.com/rust-lang/rust/pull/125779.
One current issue we have with "run" button and the newly added copy code button is that if you're on mobile devices, you can't use them. I took a look at how `mdbook` is handling it and when you click on a code example, they show the buttons. I think it's a really good idea as if you want to copy the code on your mobile device, you will click on it, showing the buttons.
Feature can be tested [here](https://rustdoc.crud.net/imperio/click-code-example/foo/struct.Bar.html).
r? `@notriddle`
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