docs: add PSG EEG validation path for sleep apnea research

[2023Anita]

Summary

• Add a PSG/EEG co-validation research extension to the sleep-apnea module docs
• Describe how RuView CSI-derived breathing and apnea candidates can be compared against scored sleep-study epochs
• Clarify that PSG, HSAT, clinician scoring, and regulated monitoring equipment remain the reference standard

Motivation

The sleep-apnea module already documents AHI-style event tracking and cautions that single-night findings should be confirmed with clinical polysomnography. As a clinician with anesthesia and sleep-medicine context, I think the next useful documentation step is to show how RuView could be studied responsibly against PSG/EEG-derived ground truth.

The added section frames RuView as an experimental contactless sensing layer, not a standalone sleep diagnostic system. It also gives researchers a concrete validation path: reference sleep-study channels, CSI-derived outputs, clock alignment, epoch-level comparison, and clinician review of discordant epochs.

Verification

• git diff --check

Risk

Docs-only change. No runtime behavior, thresholds, event IDs, APIs, or module logic are changed.