Perseus demo

[arctic-hen7]

Is your feature request related to a problem? Please describe.
There's currently no demo of Perseus in action, which makes getting reliable Lighthouse scores difficult, and a demo would improve engagement and show people what Perseus is really capable of!

Describe the solution you'd like
Deploying each of the examples would be problematic right now because some can't be hosted on modern providers like Netlify yet, so I think there are two options really available. The first is to rebuild the documentation as a Perseus app and export it statically on GitHub Pages, and the second is to create a generic demonstration app. The documentation idea appeals the most, because that also provides an opportunity for creating a proper website for Perseus, which it sorely needs as it continue to grow.

Describe alternatives you've considered
We could just create a demonstration app, but that wouldn't be a real-world use case, and so any performance scores would be a little artificial.

Additional context
Add any other context or screenshots about the feature request here.

[arctic-hen7]

Okay, the website is now live!