Add documentation for auth playground display mode

[mintlify]

Add documentation for the new auth playground display mode that shows interactive playgrounds only to authenticated users, with support for group-based access control.

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Created by Mintlify agent

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Note

Low Risk
Documentation-only changes with no product logic or behavior modifications.

Overview
Adds documentation for an auth playground display mode that hides the interactive playground for unauthenticated users (and optionally for users outside required groups) while keeping the endpoint page publicly visible.

Updates the playground configuration reference to include display: "auth", and adds examples for setting playground: auth via MDX frontmatter and via OpenAPI x-mint.metadata (including groups and public).

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