docs/guide-ja/structure-assets.md

- prepared for translation [ci skip]
- docs/guide-ja/structure-widgets.md - changed "コンテント" to "コンテンツ" [ci skip]
- docs/guide-ja/* - changed "ドキュメンテーション" to "ドキュメント" [ci skip]
- docs/guide-ja/intro-yii.md - inserted the Chinese character "易" [ci skip]
- docs/guide-ja/structure-assets.md - WIP [ci skip]
- docs/guide-ja/structure-assets.md - completed [ci skip]

close #6037