Docusaurus migration: recreate missing diff code highlighting / compare / copy feature

In the current 11th-based site, code marked with `diff:` have both language highlighting and line-by-line diff highlighting. You can also switch between two tabs; one of which shows the diff and the other of which shows the final code with the diff applied (and the copy button only appears on the latter).

This will require an extension to be written, but I already vibe-wrote it as part of #574 (hasn't been tested though).

Review the vibe-written implementation referenced in #574 and compare it with the current 11th-based site's behavior. Verify that diff-marked code retains language and line-by-line diff highlighting, provides diff and final-code tabs, and shows the copy button only on the final-code tab.

Written by the indexing model from the issue text.