docs(sveltekit): use `auth` over `getSession` (#9874)
fix: use `auth` over `getSession`
Replaces `getSession` with `auth` given that `getSession` is
now deprecated.
Refer to: https://github.com/sveltejs/kit/discussions/5883 for context.
The following PR also serves as context for the same
milestone: https://github.com/sveltejs/kit/pull/5946
