docs: add warning about sample CGM data format limitations

[surfdoc]

Summary

Addresses part of #60 by adding a prominent warning to the CGM tutorial explaining that the sample data does not fully comply with IEEE 1752/Open mHealth standards.

Problem

The sample CGM data in the tutorial (sourced from PLOS Biology research) has several format issues:

• Units: Uses MGDL instead of mg/dL
• Modality: Shows self-reported instead of sensed (CGM devices sense, not self-report)
• Temporal relationship: Uses invalid value unknown instead of OMH-compliant values
• External datasheets: Lacks proper device provenance (model, firmware)

These issues could mislead users learning the OMH format or suggest that validation isn't properly enforced.

Changes

tutorial/tutorial-cgm.ipynb:

• Added warning cell after the intro explaining the data format limitations
• Links to the Add Data Source Type guide for proper schema validation
• Makes clear this is demonstration data, not production-ready format

Related Issues

• Addresses Glucose data not in OMH format #60 (documentation portion)
• Platform investigation issue will be created separately in jupyterhealth-exchange repo

Testing

• ✅ Ran npm run myst -- build --html locally
• ✅ Build completed successfully with no errors
• ✅ Warning renders properly in MyST markdown

Next Steps

A separate issue will be created in jupyterhealth-exchange requesting:

1. Investigation of why non-compliant data exists
2. Strengthened schema validation
3. Generation of properly formatted sample CGM data

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Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com