Explain what and how to perpare for url change of JLU-GitLab

We need a text that helps users to change all urls that link to gitlab.test.uni-giessen.de and replace it with gitlab.ub.uni-giessen.de

This includes at least the following points (from @jkeyser on gitlab.com):

• they have to change the remote entry for every single clone (If its named origin: git remote set-url origin <NEW-URL>)
• every URL/link in any file (internal and external) must be changed to work

These changes also effect our support pages!