fix(docs): fix Diátaxis mode violations in concepts and how-to

[beonde]

Summary

Fix Diátaxis framework violations by separating reference-mode content from concept and how-to pages.

Documentation Audit Reference

Phase: P2-3
Severity: Medium

Changes

concepts/validation.md (rewritten)

• Before: 420-line reference document with error codes, output format specs, performance benchmarks, internal process docs
• After: 60-line concept page explaining what validation is, what categories are checked, and when to validate
• Moved content: Full reference → reference/cli/validation.md

how-to/security/policy-enforcement.md (trimmed)

• Removed: Configuration reference table (12 env vars), PDP request/response JSON format
• Moved to: reference/server/policy-enforcement.md
• Kept: All step-by-step procedural content (Options A/B, break-glass setup, verification)

New reference pages

• reference/cli/validation.md — Complete validation internals, error codes, output formats
• reference/server/policy-enforcement.md — PEP/PDP config, enforcement mode matrix, request format, telemetry fields

Navigation

• Added Validation Reference under CapiscIO Core in nav
• Added Policy Enforcement under Server API in nav

Verification

• mkdocs build --strict passes
• No content lost — all reference material preserved in new locations
• Cross-links between how-to ↔ reference pages working

[github-actions]

✅ Documentation Build Successful

The documentation build completed successfully and passed validation checks.

• ✅ Build completed without errors
• ✅ Critical files present (index.html, sitemap.xml, robots.txt)
• ✅ Link validation completed

This PR will deploy to dev-docs.capisc.io when merged to main.