[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 View system
===============
This chapter aims to describe the concept of a `view` used all along
the development of a web application and how it has been implemented
in |cubicweb|.
.. toctree::
:maxdepth: 3
views
basetemplates
primary
reledit
baseviews
startup
boxes
table
xmlrss
urlpublish
breadcrumbs
idownloadable
wdoc
.. editforms
.. embedding