Adds CI/CD for staging Cluster

This adds the capability for CI/CD in the staging environment. The gitlab-ci.yaml builds an image and pushes it to the gitlab registry. In the second stage the image tag in the values.yaml inside of the manifest directoryl gets updated. ArgoCD in the staging cluster will then sync the new state.

The manifest directory is added, which contains the necessary files to deploy in a Kubernetes Cluster.

Before merge:

• We need to adjust in the gitlab-ci.yaml the branch names from ci to main.
• Need to wait for final clearance of the subdomain
• Add probes (startupProbe, livenessProbe, readinessProbe)
• Rebase & Squash commits