Use dependency groups (update Poetry to v1.2 and Sphinx to v5)

[Cito]

Currently we cannot upgrade to Sphinx v5, because Sphinx and Flask are installed together in the dev environment, and the current versions Sphinx 5 and Flask 4 cannot coexist (see for instance here).

The best solution will be to use dependency groups to separate at least the "docs" environment from the "dev" environment. This is something that is supported and recommended in the next version 1.2 of Poetry.

I think we should do that when Poetry 1.2 has been released and Sphinx 5 is more widely used (to be sure that the themes and "read the docs" support it).