Start versioning the platform docs as well

[fhennig]

At the moment the latest stable and current nightly docs for the platform docs have not diverged and it is okay to not have versions, but it should be versioned in the long term.

Like with the operators, it needs to have tags and versions at those tags. But there is no release script, so it can be adapted manually.

• for the current docs, the version 22.06 needs to be put into the version key in the antora.yaml. This needs to be committed and tagged with docs/22.06
• afterwards, another commit should set the version key to nightly and prerelease: true
• in the antora-playbook.yaml the tags: docs/* needs to be added to the documentation repository, so the tagged versions are pulled in

Open question: Should there be an automated mechanism to release new versions? To release a new version, the version and prerelease keys in antora.yaml need to be adapted and the commit needs to be tagged. then a new commit needs to be made to set the version back to nightly and the prerelease to true. (This could be put into a script, possibly in the makefile?)