Migrate `dev` and `docs` from optional dependencies to PEP 735 dependency groups

[lochhh]

Currently, dev and docs are defined under [project.optional-dependencies] in pyproject.toml. This conflates two distinct concepts:

Optional dependencies are intended for end users who want optional features of the package (e.g. pip install movement[napari]).

Dependency groups (PEP 735) are intended for contributors and are not part of the package's published metadata.