[doc] Write about `site_cubicweb.py`
$ grep -B3 -r WRITEME doc/
doc/book/devrepo/cubes/layout.rst-The :file:`__init__.py` and :file:`site_cubicweb.py` files
doc/book/devrepo/cubes/layout.rst-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
doc/book/devrepo/cubes/layout.rst-
doc/book/devrepo/cubes/layout.rst:.. XXX WRITEME
It seems that it is explained nowhere what is the use of this file.