i18n(ja): update customization.mdx

[morinokami]

Description

• Update customization.mdx to match the English version.
  • Make social icon configuration more flexible #3025

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Latest commit: a126878

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Tracked Files

Locale	File	Note
ja	guides/customization.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]

I checked this PR as a japanese native speaker and LGTM!

[HiDeoo]

Thanks for the translation and thanks @Yusaku01 for the review 🙌

As a non-native speaker, I reviewed this PR using the Docs Translation Review Checklist and everything looks good to me 👍