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 example showing how to set a different default version

Replaces #4309 (English only as requested).

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Note

Low Risk
Low risk documentation-only change that clarifies how the version switcher chooses its default and introduces no runtime or schema behavior changes.

Overview
Adds documentation for a default: true flag on navigation.versions entries to explicitly select the default version instead of always using the first array item.

Updates both organize/navigation.mdx (new Default version section with example) and organize/settings.mdx (documents the default boolean in the versions schema reference) to keep navigation docs and config reference consistent.

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[ethanpalm]

lgtm! just a few small edits to use the word "default" a bit less 😄

[ethanpalm]

fine to ignore the failing test. It's been flaky all day but the preview looks good and nothing in this PR should possibly break anything