Explainer: What Is Predictive Parity?

[yakew7]

Summary

Add the predictive-parity explainer (used as the example contribution topic in CONTRIBUTING). Covers equal PPV across groups and how it trades off against equalized odds and demographic parity.

Why

Completes the core fairness-metric trio alongside the existing demographic-parity and equalized-odds explainers; referenced by "Why Fairness Metrics Conflict."

Tasks

• explainers/predictive-parity.md per CONTRIBUTING §8
• Worked example + the impossibility-theorem cross-link
• Add to index.html, README.md, CONTRIBUTING.md