Add an OAuth example to the new SvelteKit SSR documentation

[coppinger]

Improve documentation

https://supabase.com/docs/guides/auth/server-side/sveltekit

https://github.com/supabase/supabase/blob/master/apps/docs/content/guides/auth/server-side/sveltekit.mdx

Describe the problem

I found it slightly difficult to figure out how to use OAuth with the new SvelteKit SSR guide as there isn't an example of how to handle it—seemingly by redirecting to data.url

ex:

if (error) {
	console.error(error);
	return redirect(303, '/auth/error');
} else {
	return redirect(303, data.url);
}

Describe the improvement

It'd be great to address this in the guide. I tried to make a PR, but couldn't quite figure out how best to approach adding this info.