Invalid documentation syntax when sphinx-design is upgraded #404

In sphinx-design==0.8.0 they have deprecated the syntax we use on the card header/footer. See description here:

https://sphinx-design.readthedocs.io/en/latest/cards.html#legacy-separator-syntax

The issue this is causing is that all tests run by the Copier template which instantiate a new instance of the template end up pulling in the most recent versions of all Python packages. This means we have test failures that are quite hard to debug as they only appear in the newly instantiated templates, which I believe are deleted when the test ends.

The simple fix for now would be to update the sphinx-design package in main, and fix the deprecation warnings.

It is probably worth thinking about whether we want to take steps to avoid this issue in future - as it means that currently any CI run against even main will fail due to a third party updating their packages.