Continuous integration

From the changelog:

Added

• GitHub workflow for continuous integration and API docs.
• Randomness plug-ins via global config randomness_lib: currently only stdlib and for_tests.

Fixed

• A bit of code rot in the Cuda backend mock cuda_backend.missing.ml.
• NPY: Compatibility with OCaml 5.2.0.
• Renamed the main package name from ocannl to neural_nets_lib, to prevent the opam linter from complaining about a confusing name.