Modernise rate limits guide

[dwigg-openai]

Summary

These changes update the base model to gpt-4o-mini, introduces improved rate limit handling techniques, and refines prompt batching for chat completions. It also includes general quality improvements.

Motivation

These updates ensure the cookbook remains accurate, useful, and aligned with the latest model capabilities.

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[MW-OAI]

@dwigg-openai left some comments to review, overall looks good

[MW-OAI]

Looks good