Initial draft of Implemnt deep research (#5774)

### What problem does this PR solve?



### Type of change


- [x] Documentation Update

docs/guides/chat/implement_deep_research.md

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+---
+sidebar_position: 3
+slug: /implement_deep_research
+---
+
+# Implement deep research
+
+Implements deep research for agentic reasoning.
+
+---
+
+From v0.17.0 onward, RAGFlow supports integrating agentic reasoning in an AI chat. The following diagram illustrates the workflow of RAGFlow's deep research:
+
+![Image](https://github.com/user-attachments/assets/f65d4759-4f09-4d9d-9549-c0e1fe907525)
+
+To activate this feature:
+
+1. Enable the **Reasoning** toggle under the **Prompt Engine** tab of your chat assistant dialogue.
+
+![Image](https://github.com/user-attachments/assets/4a1968d0-0128-4371-879f-77f3a70197f5)
+
+2. Enter the correct Tavily API key under the **Assistant Setting** tab of your chat assistant dialogue to leverage Tavily-based web search
+
+![Image](https://github.com/user-attachments/assets/e8787532-7e72-49ef-8951-169ae544512f)
+
+*The following is a screenshot of a conversation that integrates Deep Research:*
+
+![Image](https://github.com/user-attachments/assets/165b88ff-1f5d-4fb8-90e2-c836b25e32e9)