The automx2 project home is GitLab

[Ralph Seichter]

After I had to close a GitHub-based pull request today, I'd like to take
the opportunity to remind interested parties that automx2 is hosted on
GitLab (https://gitlab.com/automx/automx2).

GitHub, in contrast, is used as a read-only mirror. Unfortunately there
is no way to prevent people from creating pull requests based on the
mirrored data, so I will probably have to change it to private access.

I have removed references to the mirror from our documentation, and also
updated https://gitlab.com/automx/automx2/-/blob/master/CONTRIBUTING.adoc
a little. Hopefully this will help to avoid confusion in the future.

-Ralph