document upgrade from older versions to stable release

[Svanvith]

we need to document the steps from older releases to the latest stable release

we already have https://docs.opencloud.eu/docs/admin/maintenance/upgrade/

[Svanvith]

• clone opencloud compose like our actual setting up documentation and configure the .env with the urls etc
• shut down the docker docker compose down -v in the old repo
• start the docker in the new repo

[Svanvith]

Please add a Note, that the initial Admin Password needs to be set, but the one from the opencloud_full stack is used

[Heiko-Pohl]

created a tutorial here #527