Make the repo compliant with REUSE v3.3

[giacomd]

This PR makes the repository compliant with version 3.3 of the REUSE specification.

On top of the usual Copyright and License headers, we add:

• A dedicated header preserving the package name (IMHO not really needed but just to preserve the notices as they were):

SPDX-PackageName: "covfie, a part of the ACTS project"

• A template to make the reuse tool usage add the above dedicated header: .reuse/templates/acts-project
• A REUSE.toml file to license documentation under CC-BY-4.0 and configuration files under CC0-1.0

Lastly, we replace the license checker with the recommended reuse pre-commit hook

[stephenswat]

I'm very happy with this; just have a few minor comments. Thank you!

[stephenswat]

Thank you!