Add project repository, docs etc. URLs

In setup.cfg add:

[metadata]
description = a more detailed description
project_urls =
    Homepage = https://gitlab.kit.edu/kit/virtmat-tools/vre-middleware
    Documentation = https://vre-middleware.readthedocs.io
    Download = https://pypi.org/project/vre-middleware/#files
    Source Code = https://gitlab.kit.edu/kit/virtmat-tools/vre-middleware
    Bug Reports = https://gitlab.kit.edu/kit/virtmat-tools/vre-middleware/-/issues
    Changelog = https://gitlab.kit.edu/kit/virtmat-tools/vre-middleware/-/blob/master/CHANGELOG.md
    Credits = https://gitlab.kit.edu/kit/virtmat-tools/vre-middleware/-/blob/master/CREDITS.md
    Contributing = https://gitlab.kit.edu/kit/virtmat-tools/vre-middleware/-/blob/master/CONTRIBUTING.md