Add scrolling overflow to code examples

[agjohnson]

This introduces a top margin to the highlight class block, which doesn't seem to respond to style overrides without calling it important.

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:books: Documentation preview :books:: https://read-the-docs-website--215.com.readthedocs.build/

[agjohnson]

And not sure why this is the case exactly, but overflow can trigger some strange behaviors so I'm not completely surprised:

(1rem = 16px on our website)