iac-security-scanner: Add aggregate severity stats to provider README files

[ethanolivertroy]

Summary

Each provider README (e.g., references/aws/README.md) lists resources with per-resource severity counts but lacks an aggregate summary at the top.

Proposed Change

Add a summary block at the top of each provider README with total counts:

# AWS Security Policies

Total policies: 305

**Severity breakdown:** 🔴 120 HIGH | 🟡 140 MEDIUM | 🟢 45 LOW

This gives a quick-glance view of the provider's security posture coverage without scrolling through individual resources.

Impact

• Improves discoverability and scanning efficiency
• No breaking changes — additive only
• Low effort — generate_provider_index() in parse_terrascan_policies.py already computes these counts