docs: Complete dashboard documentation with API reference and cross-links

Phase 6 Task 3 - Complete documentation updates:

1. Created comprehensive API documentation (API_DASHBOARD.md):
   - Portal dashboard endpoint (/dashboard) - full specification
   - Agent dashboard endpoint (/agent-dashboard) - full specification
   - Complete request/response examples with all fields documented
   - Error handling patterns and HTTP status codes
   - Security model and performance characteristics
   - Usage examples with JavaScript code snippets
   - SLA metrics explanation (TTO/TTR warnings and breaches)

2. Updated PLAN_CI_BROWSING.md:
   - Added "Related Features" section for Dashboard Widgets
   - Documented integration points between CI browsing and dashboard
   - Explained dual-widget architecture and visibility rules
   - Cross-referenced PLAN_DASHBOARD.md and API_DASHBOARD.md
   - Noted future CI integration possibilities in dashboard

3. Updated PLAN_DASHBOARD.md:
   - Marked API endpoint documentation as complete
   - Marked PLAN_CI_BROWSING cross-reference as complete
   - Updated Phase 6 documentation task statuses

4. Updated docs/README.md:
   - Added all three dashboard docs to Documentation Structure:
     * PLAN_DASHBOARD.md (Planning & Architecture)
     * API_DASHBOARD.md (API & Data Integration)
     * DASHBOARD_IMPLEMENTATION_SUMMARY.md (Feature Specifications)
   - Created new "Dashboard Widgets Feature" status section
   - Added Dashboard Widgets to Key Features Overview (top position)
   - Updated CI Browsing status (Phases 1-7 complete)

Documentation now provides:
- Complete API reference for both dashboard endpoints
- Clear feature relationships and integration patterns
- Comprehensive cross-referencing between related docs
- Up-to-date status tracking across all phases
- Easy navigation for developers and users

Phase 6 Documentation Status:
✅ README dual-widget documentation (main README.md)
✅ API endpoint documentation (API_DASHBOARD.md)
✅ Cross-references in PLAN_CI_BROWSING.md
✅ Documentation index updated (docs/README.md)

Author: LexioJ