Improve deprecation styling

[orhun]

We currently use diffs for deprecated elements in highlights like so:

- Axis::default().labels(vec!["a".into(), "b".into()])
+ Axis::default().labels(["a", "b"])

I was reading goreleaser docs (don't ask why) and realized they have this special tab view for old/new variants of deprecated elements:

See https://goreleaser.com/deprecations/#build

We can probably do something like this as well :)

[joshka]

This would require tabs support in GitHub flavored markdown. There's a feature request that's been open for a few years for that https://github.com/orgs/community/discussions/14964

We could probably special case it for the website, but it would still need a markdown friendly format for general use.