Add manual endpoint navigation example to MDX setup guide

[mintlify]

Added a concrete example showing how to add manual API endpoint pages to docs.json navigation structure. This clarifies the previously vague step 4 by demonstrating how to organize endpoints into groups within a tab structure.

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Created by Mintlify agent

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Note

Adds concrete navigation examples for manual API pages and documents how to reference OpenAPI endpoints in navigation.

• Docs (api-playground/mdx-setup.mdx):
  • Navigation setup:
    • Replaces vague instruction with concrete docs.json example for organizing endpoint pages under tabs -> groups -> pages.
    • Notes page-path-to-MDX-file mapping.
  • OpenAPI in navigation:
    • Adds examples to reference endpoints directly via OpenAPI file + method/path in pages.
    • Shows setting a default openapi per group with nested overrides.
    • Links to OpenAPI setup guide.

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[ethanpalm]

One small change, but lgtm