change sphinx-panels for sphinx-design

[OriolAbril]

Move from sphinx-panels to sphinx-design. The only page using sphinx panels is the books+videos one which needs to be modified to match the new design by @martinacantaro, so the change of extension should be straighforward: delete the book+videos page, change the extension used and directly use sphinx-design in the newly created pages.

Reference: https://sphinx-design.readthedocs.io/en/sbt-theme/