Add padding to CodeExample

[primer]

Add padding to CodeExample

[simurai]

Another cool thing and I remember chatting with @shawnbot about it. Being able to "pass in" any class or maybe even inline styles through the code fence declaration.

  ```html class="text-center" style="min-height: 100px;"
  <details>

Would output:

<iframe>
  ...
  <div class="p3 text-center" style="min-height: 100px;">
    <details>

This would allow us to hide the "setup" styles from code examples. And avoid being copy&pasted by mistake.

[emplums]

@simurai can you remind me where you saw this bug popping up again?

[simurai]

On the Primer CSS buttons page, after clicking on a button, the "blue focus" outline should be visible:

Actual	Expected

Another example is the Label--outline on the Primer CSS labels page. They seem to have a negative top/bottom margin:

Which hides the top/bottom border when in an iframe.

Actual	Expected

[shawnbot]

@simurai I'd like to look at the fenced code blocks prop passing today. IIRC it's a pretty simple tweak.