Update `sphinx-panels` to `sphinx-design`

[choldgraf]

Context

sphinx-panels is currently what defines our more "UI-ish" elements, but it has been superceded by sphinx-design and so we should update Jupyter Book to use this instead.

Here are our docs for panels functionality

Implementation details

• Here's an issue tracking some changes from sphinx-panels to sphinx-design: https://github.com/executablebooks/sphinx-design/issues/51
• Here's the documentation for migrating from sphinx-panels: https://sphinx-design.readthedocs.io/en/furo-theme/get_started.html#migrating-from-sphinx-panels

Tasks to complete

• [x] Confirm that the two packages have feature parity for the things we have documented in Jupyter Book
• [x] Swap out sphinx-design for sphinx-panels in the Jupyter Book docs
• [x] Update our docs for whatever new syntax it uses
• [x] Update our tests for whatever new outputs it creates
• [x] Figure out what kind of deprecation pathway we want
  • One idea: provide documentation for how users could disable sphinx-design and enable sphinx-panels and then remove that after a release or two.

This is now implemented

Check out the docs and links for migration guides

• here: https://sphinx-design.readthedocs.io/en/latest/get_started.html#migrating-from-sphinx-panels
• and here: https://github.com/executablebooks/sphinx-design/issues/51

[choldgraf]

@chrisjsewell to best approximation, is sphinx-design at a point where we could give this a shot and add it in to Jupyter Book, or do you imagine the API changing? Trying to figure out if it is "ready for active use" from your perspective

[melissawm]

~~Hey folks! Is there a plan for a "migration guide" for the elements previously provided by sphinx-panels and how they might now work with sphinx-design?~~

EDIT: Found it here https://sphinx-design.readthedocs.io/en/latest/get_started.html#migrating-from-sphinx-panels 😄