Docs: add WFGY 16-problem checklist as RAG troubleshooting reference (langgenius/dify#32439)

[onestardao]

This PR implements the documentation part of langgenius/dify#32439.

Changes:

• Adds a short “Troubleshoot RAG quality” section to the Knowledge overview page.
• Briefly introduces the 4 regions and 16 common failure patterns (input/retrieval, reasoning/planning, state/context, infra/deployment).
• Adds an external “Read More” link to the open-source WFGY ProblemMap (16 real-world RAG failure modes and suggested fixes).

No product or API changes are included in this PR.
The checklist is optional and can be used as a shared vocabulary when debugging RAG pipelines in Dify.

[dosubot]

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