Migration to sphinx-design

[chrisjsewell]

Eventually, it is envisaged that this package will be deprecated in favour of https://sphinx-design.readthedocs.io/en/sbt-theme/. See https://sphinx-design.readthedocs.io/en/furo-theme/get_started.html#migrating-from-sphinx-panels for the major changes

Feedback is welcome at https://github.com/executablebooks/sphinx-design/issues/6