Spruce demo page

[jonathonherbert]

What does this change?

Spruce the demo page, making it more usable, and a bit more inviting.

Before	After

Neater, no?

(This uses the Source design system, as that was already present when we created the elements, but is not an endorsement of using Source for tooling — the Journalism stream are anticipating taking another approach shortly)

How to test

npm run start and take a look at the demo page.