markdown: Ensure code block copy button stays in the right spot (#26074)

This PR makes it so the copy button on Markdown code blocks stays
absolutely positioned even when scrolled:

<img width="1297" alt="Screenshot 2025-03-04 at 5 28 48 PM"
src="https://github.com/user-attachments/assets/b0d0fae9-ccd6-43c1-bef3-44d8d3c3e669"
/>

We achieve this by inserting a new parent element around both the copy
button and the code block itself so we can position the copy button
absolutely within that element.

Release Notes:

- N/A