Code quality refactor (based on scikit-hep/cookie)

[phinate]

I've gone and added all of the features from the scikit-hep cookiecutter into signax! It uses the test runner nox, which is documented in CONTRIBUTING.md, and also adds a bunch of extra pre-commit hooks, including mypy and ruff.

There's also stuff in place to do ReadTheDocs if we ever want to do any documentation :)

[phinate]

One thing that has been removed here was all the uses of def __call__(self, input, *, key): when the key wasn't used in the function. Is that something that you'd like changed back based on the way you're using the Modules?

[anh-tong]

@phinate Thank you very much for this PR. In the case of Signature, I was working with equinox.nn.Sequential requiring key. It would not be necessary. I think we can remove key.

Additionally, there seems to be an error in the workflow installation for signatory, where the installation of PyTorch is missing. Once this is fixed, the PR will be absolutely awesome. Thank you again!

[phinate]

CI passed! merging now :)