Add section about deprecated repositories and how to archive them
As this creates sometimes a bit confusion, we wanted to add a specific section in the Handbook wrt archiving of deprecated repos. This is a draft and everybody is invited to give feedback.
We could add more information about the final location of archived repos such that they should reside alongside other still active repos of the project as there is a tendency to offload such deprecated repos to some other organization e.g. on GitHub.