docs: improve api.auth documentation with complete example

[devin-ai-integration]

Summary

Improves the generators.yml reference documentation for api.auth to better explain how to specify a default security scheme across the API. Previously, the documentation only said "Authentication configuration for the API" with no details on usage.

Changes:

• Added a Note in the auth-schemes section pointing users to api.auth for applying schemes as the default
• Expanded the api.auth ParamField with a clear explanation of what it does and what values it accepts
• Added a complete two-step example showing how to define a scheme and then apply it
• Included SDK usage example showing the result
• Changed the type from "object" to "string" to accurately reflect that it accepts a scheme name reference

Review & Testing Checklist for Human

• Verify the type change from "object" to "string" is accurate for api.auth (it should accept a string referencing a scheme name)
• Check that the internal anchor links work: #auth-schemes and #override-api-authentication-settings
• Review the preview deployment to ensure the documentation renders correctly and the examples are clear

Notes

Requested by: kenny
Devin session: https://app.devin.ai/sessions/30976867fe374d578a9933819c799d71

[devin-ai-integration]

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