docs: add reference pages and cookbooks

[AmitoVrito]

Summary

• Adds 5 reference pages: environment variables, error reference, troubleshooting guide, security guide, and production guide (Environment variables and configuration reference #83-Production best practices guide #87)
• Adds 3 cookbook files: 20 RAG patterns, 15 agent patterns, 15 graph workflow patterns (Cookbook — RAG patterns reference #88-Cookbook — Graph workflow patterns reference #90)
• Adds cookbook index page
• Updates sidebars.ts to include Reference and Cookbook categories

Pages added

Reference

• docs/reference/env-vars.md — complete table of all env vars, secrets manager examples, validation helpers
• docs/reference/errors.md — every exception with causes, examples, and fixes
• docs/reference/troubleshooting.md — 12 symptoms with diagnosis steps and fixes
• docs/reference/security.md — API key management, PII detection, content filtering, OWASP AI Top 10
• docs/reference/production.md — Dockerfile, docker-compose, GitHub Actions, Kubernetes HPA, OTel tracing

Cookbook

• docs/cookbook/intro.md — cookbook index
• docs/cookbook/rag-patterns.md — 20 patterns from basic to GraphRAG, HyDE, Self-RAG, RAG Fusion, eval, cost tracking, multimodal
• docs/cookbook/agent-patterns.md — 15 patterns from minimal to multi-agent supervisor/crew, MCP, MockLLM testing, eval cases
• docs/cookbook/graph-patterns.md — 15 patterns covering linear, conditional routing, parallel fan-out, human-in-the-loop, SSE/WebSocket streaming, TypedState, Mermaid export

Test plan

• npm run build passes with no errors
• Verify all sidebar links navigate correctly in dev server
• Review code examples for accuracy

Closes #83 #84 #85 #86 #87 #88 #89 #90