chore: update documentation to oci registry

[xeniape]

Part of https://github.com/stackabletech/infrastructure/issues/142
Updates references to oci registry and adjusts documentation on images and image lookup.

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[xeniape]

Release Note

Starting with this release SDP operators, product image selection and tools, like stackablectl, use oci.stackable.tech (OCI registry) as the default image registry instead of docker.stackable.tech.
To ease the transition, operator and product images, as well as operator Helm Charts, will be pushed to both registries for this release, but only tooci.stackable.tech in subsequent releases.
To keep using the docker.stackable.tech registry during this time for products, set the spec.image.repo property to docker.stackable.tech/stackable in the respective product CR.
To configure stackablectl to pull Helm Charts from the old registry, use --chart-source repo in the operator install commands. Unfortunately, those Helm Charts are using values, which specify the repository for the images to be the OCI registry and it is not possible to edit or override those values using stackablectl.
Instead, if the image repository for the operators themselves also needs to be docker.stackable.tech, use the helm install command and provide an image.repository value to override the default one. For example for airflow-operator:

helm repo add stackable-stable https://repo.stackable.tech/repository/helm-stable/
helm repo update stackable-stable
helm install --wait airflow-operator stackable-stable/airflow-operator --version 25.3 --set image.repository=docker.stackable.tech/stackable/airflow-operator

[sbernauer]

Thanks for updating and writing the guide!

[sbernauer]

Thanks!

[lfrancke]

Do we have any harbor related issue with a release note snippet?

[xeniape]

Do we have any harbor related issue with a release note snippet?

Yes, this one 😄 #716 (comment)