Fix deprecated links to Zwackelmann
Many links in
README.md and the Sphinx docs still point to Zwackelmann URLs. Obviously, this has to be fixed.
Additionally, the old build status badge doesn't work.
The TS-GitLab will have to shut down towards the end of the year — please think about migrating your projects to GitLab.com or GitHub.
(This is still a very early message, meant to keep you informed. There will be more urgent ones in the future.)