Add docs ownership and review rules

[Kylejeong2]

Summary

• Add Kyle Jeong as CODEOWNER for docs and Markdown changes.
• Add the docs styling guide to the repo's agent instruction files.
• Configure Cubic with a file-backed docs style custom rule so PR reviews actively flag violations.

Validation

• git diff --check
• Parsed cubic.yaml locally with Ruby YAML
• Confirmed .cubic/docs-style-guide.md is under Cubic's 10,000-character custom-agent limit

[changeset-bot]

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Latest commit: 9a88f4d

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