Chore: Highlight `JSON` Code Blocks

[nelsonic]

At present we don't have any CSS Selector for JSON:

This is fairly easy to fix by adding a couple of lines to highlight.css e.g: what we already have for Elixir:

book/theme/highlight.css

Lines 83 to 85 in 49ea95d

	.language-elixir {
	background: #282c34;
	}

Todo

• Add CSS selector for JSON code blocks to theme/highlight.css

[nelsonic]

Fixed: ✅