updated the infrastructure of the codebase

[smrchanda877]

This pull request includes changes to the import paths in the app.py file to reflect the new directory structure. The changes ensure that the correct modules are imported from their updated locations.

• Moved chatbot_statistics.py into metrics folder

• Moved these three files into backend folder:

1. retriever.py
2. RAG.py
3. ddos_protection.py

Changes to import paths:

• Updated the import path for initialize_milvus and query_rag to backend.RAG from RAG.
• Updated the import path for DatabaseClient to metrics.chatbot_statistics from chatbot_statistics.
• Updated the import path for handle_rate_limiting to backend.ddos_protection from ddos_protection.

[smrchanda877]

Wait.. Don't merge yet

[smrchanda877]

Changed the answerable questions and unanwerable questions in code to match the readme and wiki

[ACraig7]

LGTM