docs: link the tutorial series to the PyHealth 2.1 Colab notebooks

[solarsys]

Summary

The tutorials page opens the notebook copies in examples/tutorials/series, which predate the 2.1.0 release (they install PyHealth from GitHub and miss the fixes since). This points the page at the maintained 2.1 Colab notebooks, the same ones listed on pyhealth.dev:

• Tutorial series 00–08: links go to the 2.1 Colab notebooks, which install pyhealth==2.1.0. The descriptions for 04 and 07 are updated: 04 compares six models, 07 adds label distributions and baseline checks, and both open on a free T4 GPU.
• New "What's new in PyHealth 2.1" section: the three short notebooks on splits and reproducibility, calibration and confidence intervals, and XGBoost/code mapping.
• Two existing RST errors fixed on the same page:
  • A mis-indented row broke the readmission examples table ("exactly one bullet list expected").
  • The commented-out "Pipeline 5" link swallowed the "Multimodal & Smart Processors" heading.

The notebooks in examples/tutorials/series are unchanged; this PR only changes docs/tutorials.rst.

Testing

• Parsed docs/tutorials.rst with docutils: no errors. The only remaining warning is the pre-existing transition at the end of the file.
• tools/check_pr_rules.py passes.

🤖 Generated with Claude Code

[github-actions]

Test Coverage

Total: 67.88% (16694 / 24594 lines)

No modified files under pyhealth/.

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