docs: translate docs/themes/theme-composition

[tacrew]

概要

docs/themes/theme-compositionを翻訳しました。

チェック一覧

• 翻訳スタイルガイド に目を通しました。
• Translation Guide に目を通しました。
• textlint を使って校正を行いました。
• 文章全体を最初から読み直して不自然な箇所が無いことを確認しました。
• Allow edits from maintainers にチェックを入れました。

補足

Ref: #1

[smorimoto]

Looks good.

[Naturalclar]

@tacrew ありがとうございます！

[gatsbot]

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[smorimoto]

Thanks!