Adds copy button to every markdown code block

What does this MR do and why?

Adds a copy button to every markdown code block. This is done by creating a custom element, copy-code, which sets up the correct DOM attributes on itself to copy the code block it is attatched to.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #21172 (closed)