docs: add vanilla Kubernetes docs

[tumido]

Description of the change

Existing or Associated Issue(s)

Additional Information

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the values.yaml and added to the README.md. The pre-commit utility can be used to generate the necessary content. Use pre-commit run -a to apply changes.
• JSON Schema template updated and re-generated the raw schema via pre-commit hook.
• List tests pass for Chart using the Chart Testing tool and the ct lint command.

[kadel]

This is the setting that I've just verified that works on IKS.
it forces the PostgreSQL run as postgres user and group (uid and guide 26) and it enables the volumePermissions initContinainer that fixes the permissions on the files in the data volume

primary:
  podSecurityContext:
    enabled: true
    fsGroup: 26
  containerSecurityContext:
    enabled: true
    runAsUser: 26
volumePermissions:
  enabled: true
  image:
    registry: registry.redhat.io
    repository: rhel9/postgresql-15
    tag: latest

[tumido]

updated

[schwesig]

/lgtm