docs: update retain extraction mode documentation

[nicoloboschi]

Summary

• document all five retain_extraction_mode values in the current docs
• clarify verbatim versus chunks behavior
• fix the retain guide links for the extraction-mode environment variable

Validation

• npm run build (from hindsight-docs/)
• ./scripts/hooks/lint.sh
• git diff --check

[strix-security]

Strix Security Review

No security issues found.

_{Updated for 3738c20.}

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