cubicweb-vcwiki #3765262 Fix and document :wiki: ReST directive [in-progress]

it presently does not handle relative links right.

prioritynormal
typebug
done in0.2.0
load0.300
load left0.000
closed by<not specified>
patch[doc] Improve the README file, in particular by documenting the `wiki` ReST directive [applied][ReST directive] Handle relative links properly in the `wiki` directive. [applied][tests] Refactor tests. [applied]