Add syntax highlighting to codeblocks in the docs

[Snailedlt]

Describe the problem

I really like diff blocks in the docs to show me the differences between two code snippets... but because there's no syntax highlighting it can become a bit hard to get a quick overview of the code blocks.

some examples from the new docs:

Describe the proposed solution

Add syntax highlighting to code blocks (this is what GitHub does in pull requests)

examples:

Alternatives considered

Display diff's side-by-side like GitHub does:

Importance

would make my life easier

[dummdidumm]

Diffs now have proper syntax highlighting

[Snailedlt]

@dummdidumm It's a bit inconsistent still though.
Like sometimes there's correct syntax highlighting for the negative diffs:

While other times there's only syntax highlighting for the positive diffs, while the negative diffs is all gray:

Examples are from the Svelte 4 migration guide: https://svelte.dev/docs/svelte/v4-migration-guide