Host nightly packages in easier-to-install places

## Summary It would be helpful to make nightly packages of the Python package available in a way that they could be installed more easily by installers like `pip`. And maybe other patterns for other languages / artifacts. Today, they are only available here as GitHub Actions artifacts ([click here for docs](https://github.com/lightgbm-org/LightGBM/blob/f72ac26123a303a93899e29fca85cee973826b82/docs/Installation-Guide.rst#versioning)). ## Motivation Allows other projects to use `lightgbm` nightlies easily in their workflows, as LightGBM does with `scikit-learn`, `scipy`, to adapt early to coming changes in `lightgbm`. And that process might result in getting early feedback and bug reports here before releases go out. That type of two-way communication has been useful for `lightgbm` and its dependencies, we should try to make it easier to establish the same thing with projects that depend on `lightgbm`. ## Description N/A ## References N/A