doc: move away translations from top-level index

In translations/ we have now a dedicated index.rst where we add/remove
references to translations. Translations are in English alphabetical
order.
Signed-off-by: NFederico Vaga <federico.vaga@vaga.pv.it>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -102,29 +102,13 @@ implementation.
 
    sh/index
 
-Korean translations
--------------------
+Translations
+------------
 
 .. toctree::
-   :maxdepth: 1
-
-   translations/ko_KR/index
-
-Chinese translations
---------------------
-
-.. toctree::
-   :maxdepth: 1
-
-   translations/zh_CN/index
-
-Japanese translations
----------------------
-
-.. toctree::
-   :maxdepth: 1
+   :maxdepth: 2
 
-   translations/ja_JP/index
+   translations/index
 
 Indices and tables
 ==================

Documentation/translations/index.rst

+.. _translations:
+
+============
+Translations
+============
+
+.. toctree::
+   :maxdepth: 1
+
+   zh_CN/index
+   ko_KR/index
+   ja_JP/index