i18n(ja): add markdown-content.mdx and update mdx.mdx

[morinokami]

Description (required)

• Translate markdown-content.mdx into Japanese
• Update mdx.mdx
  • Astro v6.0 #12322
  • 6.4 Minor release #13945

Related issues & labels (optional)

• Closes #
• Suggested label: i18n

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Tracked Files

File	Note
ja/basics/layouts.mdx	Localization changed, will be marked as complete.
ja/guides/integrations-guide/mdx.mdx	Localization changed, will be marked as complete.
ja/guides/markdown-content.mdx	Localization added, will be marked as complete.
ja/guides/upgrade-to/v6.mdx	Localization changed, will be marked as complete.
ja/tutorial/4-layouts/2.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.

[Yusaku01]

LGTM!