Document composable docs.json with $ref support

[mintlify]

Summary

• Adds a new "Composable configuration with $ref" section to the global settings page
• Documents basic usage, nested references, and rules/limitations for splitting docs.json into multiple files

Relates to mintlify/mint#6584.

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Note

Low Risk
Documentation-only change that adds guidance for $ref usage in docs.json, with no runtime or configuration parsing logic modified.

Overview
Adds a new “Composable configuration with $ref” section to organize/settings.mdx describing how to split docs.json into multiple local JSON files.

Includes examples for basic and nested $ref usage, notes how sibling keys override/extend referenced objects, and documents key constraints (relative local paths, circular ref detection, project-root path restrictions, and merge behavior).

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