[exp-branch] Update documentation

Documentation needs to reflect:

• Upgraded Keycloak version
• How to add self-hosted GitLab instances
• Current limitations regarding GitLab

Documentation has to be in general proof-read and checked if it actually (still) works and results in an up and running LEAF-Writer instance. Besides documenting the workflow for (self-hosted) GitLab instances, the code has also to be improved to reduce the number of direct code changes needed to add a GitLab instance as much as possibly, at best to zero.