docs: add user journeys, security framework mapping, and docs README

[p-rog]

Summary

• Add User Journeys document with persona-based journeys (Security Architect, Platform Engineer, Compliance Officer, Application Developer) mapping ZTVP capabilities to real-world security requirements
• Add Security Framework Mapping document mapping ZTVP capabilities to industry security framework controls (NIST SP 800-207, NIST SP 800-53, CISA Zero Trust Maturity Model, EO 14028)
• Add docs/README.md as a central index for all engineering documentation, organized into Strategy & Planning, Operations, Component Guides, Use Cases, and Advanced Topics

Test plan

• Verify all links in README.md resolve correctly
• Review User Journeys content for accuracy
• Review Security Framework Mapping for completeness

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[sabre1041]

This is really good @p-rog ! Only suggestion is to rename ArgoCD -> Argo CD

Then, we'll integrate it in