docs: add quick RAG triage checklist to RAG troubleshooting page

[onestardao]

Summary
this PR adds a small “Quick RAG Triage Checklist” section to the existing RAG troubleshooting page. it gives users a symptom first table that maps what they observe to the most likely failure area, and points them to the relevant sections already documented on the same page.

Why
RAG issues are often reported as “hallucination” but the root cause is usually context side: ingestion, extraction, chunking, embeddings, retrieval, or context window limits. a short triage table helps users and maintainers converge faster, reduces repetitive support threads, and improves the signal quality of bug reports.

What changed
added a single section near the top of the RAG page:
Quick RAG Triage Checklist
a compact table (P01–P12) mapping symptom to likely pattern and which section to read first.
no other docs changes.

Scope
docs only
one file changed: docs/troubleshooting/rag.mdx

Testing
docs build not run locally in this PR (content only change)
no runtime behavior changes

[pr-validator-bot]

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