Create and Test Upgrade Guide for OpenCloud 9.x #1233

• Create the upgrade guide based on the previous Stable Release.
• Document breaking changes, prerequisites, configuration changes, and migration requirements.
• Include the supported upgrade procedures.
• Test the documented upgrade steps on an existing installation.
• Document post-upgrade verification and troubleshooting.

Existing documentation: [Upgrade 8.x.x](https://docs.opencloud.eu/docs/next/admin/maintenance/upgrade/upgrade-8.x.x/)

opencloud-eu/opencloud#3815