Document default property for version navigation

[mintlify]

Summary

• Adds documentation for the default boolean property on version objects
• Explains that by default the first version in the array is used
• Includes code examples showing how to set a different default version
• Updates English, French, and Chinese translations

This was discovered during support investigation where a customer's llms-full.txt wasn't generating because their first version had no pages.

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Note

Low Risk
Documentation-only changes that describe a new/previously undocumented default flag for navigation versions; no runtime code or data handling is modified.

Overview
Adds documentation for selecting a default docs version in navigation.versions via a default: true flag (otherwise the first entry is used), including JSON examples.

Updates the docs.json reference (settings.mdx) to list the default boolean on version objects, and propagates the same content across English, French, and Chinese docs (with minor wording/quote fixes in zh).

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