Add code syntax highlighting support for Markdown

[katjuell]

Add multi-language syntax highlighting support for code blocks that use the <code> element. Work with Brand to come up with a theme or a set of syntax highlighting colours that we can use.

This page is an example that includes a few code blocks but the colours are too bright. View this thread for context.

[katjuell]

We currently use a plugin for this in our Gatsby project.

There's an analogous ecosystem in Next land that we can hook into as well; rehype-prism, for example, which Learn used to have, or prism-react-renderer, which we switched to in Learn so we could have author-added highlighting (where authors can highlight certain parts of the code for emphasis): relevant PRs here and here

[st0nebreaker]

Closed in PR#5520