docs: Add Fireworks.AI and Together.AI examples

[leowalker89]

This PR adds examples for using Fireworks.AI and Together.AI with PydanticAI to the OpenAI-compatible Models section of the documentation.

Changes:

• Added a section for Fireworks.AI with instructions on how to get an API key and use it with OpenAIModel
• Added a section for Together.AI with instructions on how to get an API key and use it with OpenAIModel
• Included links to the model libraries for both providers
• Formatted examples to match the existing style and pass Black formatting checks

I've tested both examples and verified they work correctly with the respective APIs.

These additions help users easily integrate with these popular model providers using the OpenAI-compatible interface.

[hyperlint-ai]

PR Change Summary

Added examples for integrating Fireworks.AI and Together.AI with PydanticAI in the documentation.

• Introduced a section for Fireworks.AI with API key instructions and usage examples.
• Introduced a section for Together.AI with API key instructions and usage examples.
• Included links to model libraries for both Fireworks.AI and Together.AI.
• Formatted examples to align with existing documentation style.

Modified Files

• docs/models.md

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[Kludex]

Thanks.