Improve docs

[Emmanuel-Melon]

The documentation isn't very helpful when it comes to deciding whether to create a single instance for the app or use multiple instances. It's also unclear where to set the client, and the example only covers a single page. So, if I have multiple pages, would I need to set it for each one, including +layout.svelte?

For instance, the React client lets you wrap your components in ApolloProvider instance.