Add PR preview deployments for web app

[issy]

As far as I can tell, GitHub Pages doesn't have good support for preview deployments, which isn't great DX. There are a couple of free alternatives with preview deployments that aren't so clunky.

• Cloudflare Pages
• Netlify
• Vercel

Since the web app is just a static site, these options should all be free.
There might also be a good route for preview deployments with GH Pages, so I'll investigate that as well.

[issy]

This action looks promising. https://github.com/marketplace/actions/deploy-pr-preview