Simplify + update rate limit docs

[billytrend-cohere]

This PR updates the documentation for Cohere API rate limits, introducing a new distinction between evaluation and production keys. It provides a clear overview of the rate limits for each endpoint, ensuring users understand the limitations and capabilities of each key type.

Changes:

• API Key Types: The PR replaces the term "trial keys" with "evaluation keys" and introduces "production keys". Evaluation keys are free but limited in usage, while production keys are paid and have no usage restrictions.
• Rate Limits Table: A new table is added, detailing the rate limits for various endpoints with evaluation and production keys. This provides a concise overview of the limits for each endpoint.
• Evaluation Key Limitations: A new section is added, outlining the limitations specific to evaluation keys. It clarifies that organizations can have unlimited evaluation keys, and these keys share the rate limits mentioned in the table.
• Removed Sections: The previous sections on "Trial Key Limitations" and "Production Key Specifications" have been removed, as the new table and evaluation key section cover the relevant information.

[github-actions]

🌿 Preview your docs: https://cohere-preview-c59f5b3a-2806-4443-9052-65a0867f1f7f.docs.buildwithfern.com

[github-actions]

🌿 Preview your docs: https://cohere-preview-af16bacc-cb34-424e-82c1-6d36844935fb.docs.buildwithfern.com

[github-actions]

🌿 Preview your docs: https://cohere-preview-ecb859f4-9297-4010-9c40-1eabd899eea8.docs.buildwithfern.com