docs: clarify structured output map limitations

[Rohan5commit]

Summary

• Clarifies the current Map limitation for structured outputs.
• Adds a concrete workaround so readers know how to model arbitrary key/value data.

Related issue

• Fixes Structured outputs Map handling has confusing error message and isn't well documented #485

Guideline alignment

• Single-file README clarification
• No generated files touched
• No behavior changes

Validation

• git diff --check

[Copilot]

Pull request overview

This PR updates the documentation to clarify that Map fields aren’t supported when deriving JSON Schema for Structured Outputs, and suggests a practical modeling workaround for representing arbitrary key/value data.

Changes:

• Documents that Map-typed fields result in a schema with no named "properties" for that sub-schema.
• Adds guidance to model arbitrary key/value data as a list of entry objects with named fields.

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