Update structured output docs for model providers

[Unshure]

Description

Adding documentation on how to build structured_output in the Custom model provider docs, and updated existing model providers with info on how to use it.

#95

Type of Change

• Content update/revision

Motivation and Context

Give users info on how to build the structured output feaature into their model providers

Areas Affected

All Model Provider docs

Screenshots

N/A

Checklist

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

Additional Notes

N/A

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