[doc] fix warnings/errors in doc build - fix links to images - fix a couple of typos - re-add IDownloadableOneLineView doc - rename documenting.rst back to .txt, it's intended as a doc of how to write rst, not part of the rst doc Related to #4832808

Cubes
=====

This chapter describes how to define your own cubes and reuse already available cubes.

.. toctree::
   :maxdepth: 1

   layout.rst
   cc-newcube.rst
   available-cubes.rst