docs: add Qwen integration guide and update qwq documentation

[TBice123123]

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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• Updating outdated or incomplete qwq documentation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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Project	Deployment	Preview	Comments	Updated (UTC)
langchain	Ready	Preview	Comment	Sep 12, 2025 2:46am

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #32817 will improve performances by 17.78%

_{Comparing TBice123123:docs/qwen (9035bd8) with master (338d3d2)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

⚡ 2 improvements
✅ 11 untouched

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_import_time[InMemoryVectorStore]	688.8 ms	610.5 ms	+12.83%
⚡	test_import_time[RunnableLambda]	560.7 ms	476.1 ms	+17.78%

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #32817 will not alter performance

_{Comparing TBice123123:docs/qwen (9035bd8) with master (338d3d2)}

Summary

✅ 14 untouched

[mdrxy]

Hello! Thank you for your patience - we've been in the middle of a docs hiatus while we port things over to the new docs site.

I'll merge this, but I'd ask that you synchronize the changes to the new docs repo, as these changes will soon become invisible as we deprecate the current docs pages.

Please view the new contributing guide for further info. Don't hesitate to ask questions, as we've rewritten parts of it and may have missed something.

I've opened an issue in the new repo - when you make the new PR, make sure to include the words Fixes #525 so that the issue is automatically closed when the changes are merged in.