Differentiate code block styling

[beechnut]

Code blocks would ideally look meaningfully different from body text. At the moment, body text (.env and docker-compose.yml) look almost the same as the code blocks, differing only in font face.

I'd expect something more visually offset, like this block from USWDS.

Definition of done

• [ ] Code appears meaningfully offset, whether through color, border, etc.
• [ ] Shows line numbers along left side (see #5 here)

[peterrowland]

PR for changes.

[bpdesigns]

@cantsin can you look at this today before @peterrowland is out of the office?