Demo analytics code

[scosman]

We could/should have a quick example of how to add analytics. We don't need to pick a provider, let's leave that to the user. But for a SvelteKit noob, we can tell them where to put it.

Tasks:

• Add comment where the analytics JS code should be injected. Probably app.html? Can be as simple as
• Add a step in README guide for replacing that comment with the tracker of your choice, with links to GA, PostHog, Plausible docs for getting your code/ID.