Add Tutorials page and fix stale Colab links

[jhnwu3]

Summary

• New tutorials.html, linked from the homepage nav. It mirrors PyHealth's docs/tutorials.rst:
  • Core API tutorials 0–7, each with Colab, video, API docs, and a Source link to its notebook in examples/tutorials/.
  • The five end-to-end pipelines, with the dataset → model each uses.
  • Links to the multimodal notebook, examples/, and the data-access notebook.
• Broken link: models.html "Open Tutorial Notebook" pointed to a Drive notebook that isn't public. It now points to Tutorial 3 (pyhealth.models).
• Duplicate notebooks: datasets.html, tasks.html and 14 entries in data/tasks.json linked to copies of Tutorials 1/2 that aren't the ones in docs/tutorials.rst. The content is identical, but they now use the canonical IDs.

Testing

• Link crawl: checked internal and external links; all 229 external links return 200, no broken internal links, and the readthedocs #anchors resolve.
• Colab notebooks: each was downloaded from Drive to confirm it's public, since Colab returns 200 even for private notebooks.
• Catalog check: scripts/check_site_consistency.py --pyhealth <PyHealth master> passes.
• Rendering: checked the page in headless Chrome at 1280px and 375px.

Merge order: sunlabuiuc/PyHealth#1257 (medcode ICD mappings) → sunlabuiuc/PyHealth#1256 (tutorial notebooks + docs links) → this PR. The Source links for the data, trainer and medcode tutorials point at notebooks updated in #1256, and the Colab medcode tutorial needs #1257. origin/site/shared-nav-footer builds the nav from scripts/sync_site_chrome.py. Whichever branch lands second should add tutorials.html to that script's NAV_LINKS and PAGES.

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