Openstack documentation is missing from KKP Supported Cloud Providers

We need to add documentation about the Openstack cloud provider in KKP Supported Cloud Providers page. xref: https://docs.kubermatic.com/kubermatic/v2.28/architecture/supported-providers.

Having a section for supported Operating Systems (along with version details), known issues, and links for new features (like the [Load Balancer configuration addition](https://github.com/kubermatic/docs/pull/2007)) would be helpful.

Start with the referenced KKP Supported Cloud Providers page and inspect how existing provider sections are structured. Add the OpenStack section with supported operating systems and versions, known issues, and links to relevant new features such as the Load Balancer configuration addition. Done means the page clearly covers these requested details and links are valid.

Written by the indexing model from the issue text.