Add MkDocs Material documentation and LLM guide

[Suke0811]

Summary

• add MkDocs configuration using the Material theme with a black palette and navigation for the docs site
• document quickstart and usage examples drawn from repository patterns
• provide an API reference covering the unified spin entry point and RateControl class and add mkdocs-material to dev dependencies
• add an LLM-focused guide page and navigation entry highlighting core behaviors and prompting tips

Testing

• not run (not requested)

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Codex Task