Document how to use Cohere models through the OpenAI provider.

[ryanycoleman]

Description

This PR adds Cohere as a model provider to the Strands documentation by describing how to use the pre-existing OpenAI model provider, as an alternative to a custom implementation proposed in strands-agents/sdk-python#236.

Type of Change

• New content addition

Motivation and Context

This approach minimizes the moving parts that need to be maintained to enable Strands to work with Cohere models over time, requiring only that Strands maintain the OpenAI provider and that Cohere maintain their compatibility layer with its API spec.

Areas Affected

https://strandsagents.com/latest/user-guide/concepts/model-providers

Screenshots

Checklist

• [ X] I have read the CONTRIBUTING document
• [ X] My changes follow the project's documentation style
• [X ] I have tested the documentation locally using mkdocs serve
• [ X] Links in the documentation are valid and working
• [ X] Images/diagrams are properly sized and formatted
• [X ] All new and existing tests pass

Additional Notes

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[ryanycoleman]

See also #108 but that example may not work without streaming disabled.