docs: add Qwen integration guide and update qwq documentation (langchain-ai#32817)

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**Description:**  
Add documentation for Qwen integration in LangChain, including setup
instructions, usage examples, and configuration details. Update related
qwq documentation to reflect current best practices and improve clarity
for users.

This PR enhances the documentation ecosystem by:
- Adding a new guide for integrating Qwen models
- Updating outdated or incomplete qwq documentation
- Improving structure and readability of relevant sections

**Issue:** N/A  
**Dependencies:** None

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Co-authored-by: Mason Daugherty <mason@langchain.dev>

Author: 2 people