[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
Edition control
===============
This chapter covers the editing capabilities of |cubicweb|. It
explains html Form construction, the Edit Controller and their
interactions.
.. toctree::
:maxdepth: 2
form
dissection
editcontroller
examples