Add project metadata for PyPI

[jameslamb]

Contributes to rapidsai/build-planning#31

Now that the project is being published on PyPI (https://pypi.org/project/rapids-build-backend/), it'd be helpful to add some links to direct people from the PyPI page back to this repo.

This does that.

Inspired by:

• https://github.com/rapidsai/dask-cuda/blob/098109aef0793b43b94f9746ed9edbde44c2d761/conda/recipes/dask-cuda/meta.yaml#L49
• https://github.com/scikit-build/scikit-build-core/blob/d9c41f9da5ddd32345f4efdfac6c96667b424b8d/pyproject.toml#L13