[Docs] Add Authentication Note and Link to Guided Tour in Quick Demo Section

The current Quick demo section in the `smolagents` README does not mention that authentication may be necessary for certain use cases, especially when using `HfApiModel`. This can lead to confusion when users encounter errors or when trying to access gated models.

### Suggested Change ###
Add a short note in the Quick demo section explaining that:

- Users may need to authenticate to avoid rate limits or access gated models.
- Link to [Guided Tour](https://huggingface.co/docs/smolagents/guided_tour) for detailed instructions on how to set the token.

### Why This is Important ###

- Helps new users avoid common errors
- Ensures smoother onboarding experience