i18n(ja): Add guides/migrate-to-astro/from-svelteKit.mdx

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Description (required)

Add missing Japanese translation
This improves completeness of the Japanese docs and aligns them with the latest English source.

original source: https://github.com/withastro/docs/blob/main/src/content/docs/ja/guides/migrate-to-astro/from-sveltekit.mdx

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🔄️	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.