[IMP] developer: rename reference directories to "Py/Js framework"

It was confusing to label the directory for pages related to the Python
framework "Backend" as, in Odoo, the backend is the web client, and the
frontend is the portal/website. It also led to the "Standard modules"
directory to be placed within the "Backend" directory as of `saas-15.1`
to indicate that they were part of the backend too, but that was a
mistake. Indeed, most standard modules comprise JavaScript methods, and
we could want to document these in the reference at some point.

X-original-commit: 8046f742bc
Part-of: odoo/documentation#2457

content/developer/reference.rst

@@ -1,5 +1,4 @@
 :nosearch:
-:types: reference
 
 =========
 Reference

content/developer/reference/backend.rst

@@ -1,8 +1,8 @@
 :nosearch:
 
-=======
-Backend
-=======
+================
+Python framework
+================
 
 .. toctree::
     :titlesonly:

content/developer/reference/frontend.rst

@@ -1,8 +1,8 @@
 :nosearch:
 
-========
-Frontend
-========
+====================
+JavaScript framework
+====================
 
 .. toctree::
     :titlesonly: