Support Sphinx v9

[zerocewl]

Context

Please support Sphinx v9

https://github.com/sphinx-doc/sphinx/releases/tag/v9.0.4

sphinx-design currently pins to <9

https://github.com/executablebooks/sphinx-design/blob/main/pyproject.toml#L31

maybe related to

#206

and

#245

Proposal

No response

Tasks and updates

No response

[chrisjsewell]

Currently blocked on executablebooks/MyST-Parser#1076