Site: Update/Fix SSR API docs

[Conduitry]

The SSR example here is at the very least highly misleading, as it doesn't show you doing require('svelte/register') and because it doesn't show that the component itself will be available as .default in the module if you do that.

We could align this better with the svelte/register docs. We could also have links between the two sections.

[akaSybe]

Finally someone explained how to use server-side API. Thank you!