[Code Embeddings] Add developer documentation for Duo Chat Integration

Context

ActiveContext has some generic usage docs, but this does not cover everything for the Code Embeddings Indexing and its use in Duo Chat.

We need to add developer docs specific to:

• Code Embeddings Indexing Pipeline - addressed in #571911
• Duo Chat Integration - this issue
  • /include <repository|directory> commands
  • Integration in Classic Chat
  • Integration in Agentic Chat: Codebase Semantic Search tool