docs: add conditional query execution for authenticated users

[mmailaender]

This PR adds a one-liner that shows how to use the new skip query feature from convex-svelte to conditionally execute a query if a user is authenticated or not.

This solves a common question/problem and avoids a major effect in teardown error that happens if the user tries deriving from a query to run a query like so conditionally in a dynamically mounted component (Dialog, popup, etc.)(More explained in the convex-svelte docs):

// Bad
$derived( isAuthenticated ? useQuery(api.users.queries.getActiveUser, {}) : {} );  

// Good (Enabled with convex-svelte 0.0.12
useQuery(api.auth.getCurrentUser, () => (isAuthenticated ? {} : 'skip'));

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