Add DocumentDB Search and RAG notebook labs

[Khelan Modi (khelanmodi)]

Summary

• Add notebook-first DocumentDB Search module with before/after labs for Python, C#, and Node.js
• Add notebook-first DocumentDB RAG Pipeline module with before/after labs for Python, C#, and Node.js
• Include OpenAI embedding generation, DocumentDB vector search, full-text search, fuzzy/phrase search, hybrid RRF, and grounded prompt exercises

Notes

• before notebooks are guided student exercises with TODOs
• after notebooks are completed reference solutions
• Participants provide an Azure DocumentDB connection string and OpenAI API key