i18n(zh-cn): update the chinese transition of guide of upgrade to v6

[mitian233]

Description (required)

New: chinese transition of guide of upgrade to v6

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Pull request overview

Adds the Simplified Chinese (zh-cn) version of the Astro “Upgrade to v6” guide, aligning the zh-cn documentation set with the existing English v6 migration content.

Changes:

• Added a full new zh-cn guide page for upgrading projects from Astro v5 to v6.
• Included localized explanations for dependency upgrades, deprecated/removed features, changed defaults, and breaking changes.
• Wired the page into Starlight via frontmatter/sidebar metadata and shared MDX components.

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Tracked Files

File	Note
zh-cn/guides/upgrade-to/v6.mdx	Localization added, will be marked as complete.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[mitian233]

keep draft and wait for the rest to be translated