Improve contrib instructions for Freedesktop projects
Current instructions on how to send translation from D-L to Freedesktop's Gitlab doesn't help much a less experienced translator. It points to the project's merge request page... but there is nothing to do in there. It isn't like the translator can "file a merge request" and upload the PO file in there.
My proposal is to point to How to fork a project and How to create a merge request pages from Gitlab documentation, and also the target project's main page (instead of merge request, which doesn't have a fork button).
targetproject is the project name linked to the main page of its repository.