[MOV] developer: move assets doc into frontend

Now that the reference is split into backend/frontend, it makes more
sense to have the assets page into the frontend section.

closes odoo/documentation#1243

Signed-off-by: Géry Debongnie (ged) <ged@openerp.com>

content/developer/reference/backend.rst

@@ -17,4 +17,3 @@ Backend
     backend/testing
     backend/http
     backend/mixins
-    backend/assets

content/developer/reference/frontend.rst

@@ -8,6 +8,7 @@ Frontend
     :titlesonly:
 
     frontend/framework_overview
+    frontend/assets
     frontend/javascript_modules
     frontend/owl_component_system
     frontend/registries