Add config for Japanese translation.

I weight 6 on ja-jp because I saw a PR for korean translation on Gitea
repository. If there is no problem on that PR, it will be merged, and
I expect that the weight of ja-jp will be 6.

Signed-off-by: TAKAHASHI Shuuji <shuuji3@gmail.com>

config.yaml

@@ -300,4 +300,51 @@ languages:
         - name: 포럼
           url: https://discourse.gitea.io/
           weight: 80
-          pre: group
+          pre: group
+
+  ja-jp:
+    weight: 6
+    languageName: 日本語
+    menu:
+      page:
+        - name: ウェブサイト
+          url: /ja-jp/
+          weight: 10
+          pre: home
+          post: active
+        - name: ドキュメント
+          url: https://docs.gitea.io/ja-jp/
+          weight: 20
+          pre: 質問
+        - name: API
+          url: https://try.gitea.io/api/swagger
+          weight: 45
+          pre: plug
+        - name: ブログ
+          url: https://blog.gitea.io/
+          weight: 30
+          pre: rss
+        - name: コード
+          url: https://code.gitea.io/
+          weight: 40
+          pre: code
+        - name: 翻訳
+          url: https://crowdin.com/project/gitea
+          weight: 41
+          pre: language
+        - name: ダウンロード
+          url: https://dl.gitea.io/
+          weight: 50
+          pre: download
+        - name: GitHub
+          url: https://github.com/go-gitea/
+          weight: 60
+          pre: github
+        - name: Discord チャット
+          url: https://discord.gg/Gitea
+          weight: 70
+          pre: comment
+        - name: フォーラム
+          url: https://discourse.gitea.io/
+          weight: 80
+          pre: group