LangGraph Mail Sorting notebook & course page update

[karenwky]

• update notebook according to course page
• fix import missing START from langgraph.graph
• update course page according to notebook (Langfuse code block)

[sergiopaniego]

Thanks for the contribution!

Could you give more details about the issue addressed in this PR?
As pointed in the previous PRs, the current source of truth lives for the notebooks in the Hugging Face organization here: https://huggingface.co/agents-course/notebooks.

If you want to propose and improve or find and issue in the notebooks, we’d be more than happy to review and accept a contribution in the HF Org.

[karenwky]

I noticed quite a few HF Org notebooks are outdated compared with the course page, and even bugs are fixed in GitHub notebooks but not HF Org notebooks.

The issue is there are 3 versions of notebook: course page is the most updated content; HF Org version no update but may have some extra content compared with course page; GitHub version only update on bug fix.

There are quite a few PR on HF Org about notebooks, I guess I may check again for this issue after all those PR are completed.