[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
The 'download' views
====================
.. automodule:: cubicweb.web.views.idownloadable
Components
----------
.. autoclass:: cubicweb.web.views.idownloadable.DownloadBox
Download views
--------------
.. autoclass:: cubicweb.web.views.idownloadable.DownloadView
.. autoclass:: cubicweb.web.views.idownloadable.DownloadLinkView
.. autoclass:: cubicweb.web.views.idownloadable.IDownloadablePrimaryView
.. autoclass:: cubicweb.web.views.idownloadable.IDownloadableOneLineView
Embedded views
--------------
.. autoclass:: cubicweb.web.views.idownloadable.ImageView
.. autoclass:: cubicweb.web.views.idownloadable.EHTMLView