i18n(zh-cn): Add content-loader-reference.mdx & Update related docs

[Nin3lee]

Description (required)

• Add content-loader-reference.mdx
• Update content-collections.mdx
• Update astro-content.mdx

Related issues & labels (optional)

• 5.0.0 beta docs #9240
• Updated docs to reflect that getEntry returns undefined #10688
• Content Loader API: include docs for the built-in loaders #10710
• Suggested label: i18n

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File	Note
zh-cn/guides/content-collections.mdx	Localization changed, will be marked as complete.
zh-cn/reference/content-loader-reference.mdx	Localization added, will be marked as complete.
zh-cn/reference/modules/astro-content.mdx	Localization changed, 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.

[liruifengv]

存储好像是对的，忽略我那部分建议 @Nin3lee

[liruifengv]

LGTM!