Add project repository, docs etc. URLs

In setup.cfg add:

[metadata]
project_urls =
    Homepage = https://gitlab.kit.edu/kit/virtmat-tools/vre-language
    Documentation = https://vre-language.readthedocs.io
    Source Code = https://gitlab.kit.edu/kit/virtmat-tools/vre-language
    Changelog = https://gitlab.kit.edu/kit/virtmat-tools/vre-language/-/blob/master/CHANGELOG.md
    Credits = https://gitlab.kit.edu/kit/virtmat-tools/vre-language/-/blob/master/CONTRIBUTING.md
    Bug Reports = https://gitlab.kit.edu/kit/virtmat-tools/vre-language/-/issues