[docs] Request: Add a getter sample for reactivity in the Svelte Quickstart

[aster-void]

Code Example:

<script lang="ts">
  let userId = $state(1);

  // this query is NOT reactive to userId.
  const userQuery = useQuery(api.users.get, { id: userId });
  
  // this query IS reactive to userId.
  const userQuery = useQuery(api.users.get, () => ({ id: userId }));
</script>

ID: <input bind:value={userId} />
-> Name: {userQuery.data?.name ?? "Loading..."}

Context:

Because of how reactivity works in svelte 5, states references in a query must be wrapped in a getter function. Otherwise, the query won't change after userId changes.

Request:

Please add this example (or similar one) to Svelte Quickstart. It's so critical that the convex-svelte's README is not enough.