guide ja

- docs/guide-ja/structure-extensions.md - prepared for translation [ci skip]
- docs/guide-ja/structure-assets.md - small changes [ci skip]
- docs/guide-ja/structure-extensions.md - WIP [ci skip]
- docs/guide-ja/structure-extensions.md - completed [ci skip]
- docs/guide-ja/structure-widgets.md - broken link fixed [ci skip]

close #6062