[IMP] developer: rename addons/js => backend/frontend

The terminology did not really make sense for a lot of these sub pages.
This change should have been done long ago, but was probably delayed
because some people in Odoo call the web client the "backend", which is
really misleading.

This commit also renames the title "Reference Guides" to "Reference",
since this section does not contain any guides anyway.

Part-of: odoo/documentation#1243

content/developer/reference/addons.rst

@@ -1,20 +0,0 @@
-:nosearch:
-
-=============
-Server Addons
-=============
-
-.. toctree::
-    :titlesonly:
-
-    addons/orm
-    addons/data
-    addons/actions
-    addons/views
-    addons/module
-    addons/reports
-    addons/security
-    addons/testing
-    addons/http
-    addons/mixins
-    addons/assets

content/developer/reference/backend.rst

@@ -0,0 +1,20 @@
+:nosearch:
+
+=======
+Backend
+=======
+
+.. toctree::
+    :titlesonly:
+
+    backend/orm
+    backend/data
+    backend/actions
+    backend/views
+    backend/module
+    backend/reports
+    backend/security
+    backend/testing
+    backend/http
+    backend/mixins
+    backend/assets

content/developer/reference/frontend.rst

@@ -0,0 +1,20 @@
+:nosearch:
+
+========
+Frontend
+========
+
+.. toctree::
+    :titlesonly:
+
+    frontend/framework_overview
+    frontend/javascript_modules
+    frontend/owl_component_system
+    frontend/registries
+    frontend/services
+    frontend/generic_components
+    frontend/hooks
+    frontend/javascript_cheatsheet
+    frontend/javascript_reference
+    frontend/mobile
+    frontend/qweb

content/developer/reference/javascript.rst

@@ -1,20 +0,0 @@
-:nosearch:
-
-==========
-Javascript
-==========
-
-.. toctree::
-    :titlesonly:
-
-    javascript/framework_overview
-    javascript/javascript_modules
-    javascript/owl_component_system
-    javascript/registries
-    javascript/services
-    javascript/generic_components
-    javascript/hooks
-    javascript/javascript_cheatsheet
-    javascript/javascript_reference
-    javascript/mobile
-    javascript/qweb