Remove / reduce contribution guide from appendix
The appendix contains instructions on how to contribute to gimp-help: https://gitlab.gnome.org/GNOME/gimp-help/-/blob/master/src/appendix/contributing.xml
I believe this should be removed or rather rewritten such that it just links to the contribution guidelines in gimp-help repository. In my opinion it doesn't make sense to provide contribution instructions in multiple places, since it's duplicated work and easy to forget updating one of the places. For example, the latest additions to the documentation guidelines have not been added to the appendix. Since new contributors will unavoidably have to visit the repository, it makes sense to store the contribution guidelines there.
As a side note, I would have expected that the contribution guide in the appendix is about contributing to GIMP, not gimp-help.