[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

.. -*- coding: utf-8 -*-

.. _Part4:

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Appendixes
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The following chapters are reference material.

.. toctree::
   :maxdepth: 1
   :numbered:

   faq
   rql/index
   mercurial
   depends
   docstrings-conventions