Migrate from sphinx-panels to sphinx-design

[yousefmoazzam]

Something I saw when trying to understand why the local docs build has so many warnings (see #468) was that the sphinx-panels package was planned to be deprecated in favour of sphinx-design, see executablebooks/sphinx-panels#67 (comment).

Luckily, sphinx-design has a section in its docs for migrating from sphinx-panels to sphinx-design https://sphinx-design.readthedocs.io/en/furo-theme/get_started.html#migrating-from-sphinx-panels, so hopefully this wouldn't be too bad to do, and would free us from a deprecated dependency for the docs.

[dkazanc]

thanks for spotting this... looks like something we better do for all httomo packages then.