Use custom formatting for JSON-LD

[bergos]

The JSON-LD content is currently rendered with highlight.js, which looks different from the Turtle serialization and doesn't support highlighting features like the focus node. See the comment on the PR. We should consider switching to the same custom CSS formatting that Turtle uses to work around these limitations.

To avoid repetitive work, this editorial work should be done once all examples are finished. Turtle should be the primary format until then.