[doc] create page «what is a cube?»
Create a page with content taken from https://cubicweb.readthedocs.io/en/latest/book/intro/concepts/#cubes.
Update the corresponding link in index.rst :
What is a cube <Cube>
This page should explain what is a cube, list some useful example and may link to https://forge.extranet.logilab.fr/cubicweb/cubes (or other information)