Add versioning to the docs

[Svanvith]

Add versioning to the docs, so we have with the new production release a new version

https://docusaurus.io/docs/versioning

[Svanvith]

I added Info about versioning #501

@db-ot could you please have a look if it is ok what I wrote about the versioning

[schweigisito]

Once we have versioned docu we have to take care that the https://github.com/opencloud-eu/markdown-docs-generator tool builds the static content from the stable branches. Currently it's always built from main, but it's already configurable in https://github.com/opencloud-eu/markdown-docs-generator/blob/main/Makefile#L1