Document x-default extension for API playground

[mintlify]

Summary

• Added documentation for the x-default OpenAPI extension on security schemes, which pre-fills authentication fields in the API playground with default values
• Noted that x-default also works on any schema property as an alternative to the standard default field for playground input pre-filling

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Note

Low Risk
Documentation-only change that adds guidance for using the nonstandard x-default extension; no runtime or data-handling code is modified.

Overview
Documents the x-default OpenAPI extension for the API playground, showing how to pre-fill authentication inputs for apiKey and http bearer security schemes.

Also notes that x-default can be applied to schema properties to seed playground inputs without using the standard schema default value.

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