docs: Fixed TODO id in animation tutorial

[F2]

The tutorial on advanced animations (see this and this) is using todo.id without ever defining id.

For example, in TodoList.svelte, it says:

<li
  class={{ done: todo.done }}
  in:receive={{ key: todo.id }}
  out:send={{ key: todo.id }}
>

However, the todos are defined in App.svelte like this:

const todos = $state([
  { done: false, description: 'write some docs' },
  { done: false, description: 'start writing blog post' },
  { done: true, description: 'buy some milk' },
  { done: false, description: 'mow the lawn' },
  { done: false, description: 'feed the turtle' },
  { done: false, description: 'fix some bugs' }
]);

This PR adds the id to the todos, and also uses the id as the key in the #each loop.

Incrementing the id for new todos could be done in many ways, but I chose the uid++ way to be consistent with this example.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time.
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.

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[eltigerchino]

Thank you!