Extend our custom sphinx extension to both get the images and generate the whole gallery.rst page.
It allows dropping several directives as we now generate an rst file that is parsed by sphinx and
rendered thanks to sphinx-design, so we no longer need nbsphinx nor sphinx-gallery.
[x] PR description contains a link to the relevant issue: closes #537, closes #515, closes #409,
related to #447
Extend our custom sphinx extension to both get the images and generate the whole gallery.rst page. It allows dropping several directives as we now generate an rst file that is parsed by sphinx and rendered thanks to sphinx-design, so we no longer need nbsphinx nor sphinx-gallery.