[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

Data model
==========

This chapter describes how you define a schema and how to make it evolves as the time goes.

.. toctree::
   :maxdepth: 1

   definition
   metadata
   baseschema
   define-workflows