Add numa(aytdm) to Japanese primary maintainer(s) (nuxt#747)

* ja に足りていないファイルを追加して menu.json を更新

Add new api files to ja

Add new example files to ja

Translate ja/examples/custom-page-loading.md

* Add numa(aytdm) to Japanese primary maintainer(s)

Author: inouetakuya

README.md

@@ -40,8 +40,8 @@ Chinese translation is maintained by AOTU Labs from China, Shenzhen.
 Japanese translation is maintained by [Vue.js Japan User Group](https://github.com/vuejs-jp/home).
 
 * Translation Repo - [https://github.com/vuejs-jp/ja.docs.nuxtjs](https://github.com/vuejs-jp/ja.docs.nuxtjs)
-* Primary maintainer - [INOUE Takuya](http://blog.inouetakuya.info/), [HANATANI Takuma(@potato4d)](https://github.com/potato4d)
-* Primary translator - [INOUE Takuya](https://github.com/inouetakuya), [HANATANI Takuma(@potato4d)](https://github.com/potato4d)
+* Primary maintainer - [INOUE Takuya(@inouetakuya)](http://blog.inouetakuya.info/), [HANATANI Takuma(@potato4d)](https://github.com/potato4d), [numa(@aytdm)](https://github.com/aytdm)
+* Primary translator - [INOUE Takuya(@inouetakuya)](https://github.com/inouetakuya), [HANATANI Takuma(@potato4d)](https://github.com/potato4d), [numa(@aytdm)](https://github.com/aytdm)
 
 ### Korean