Decide the versions to index based on `_data/version.yaml`

It would seem quarkus.io uses this file to determine which versions of the documentation are available: https://github.com/quarkusio/quarkusio.github.io/blob/main/_data/versions.yaml

We should do the same, to avoid problems such as this one: https://github.com/quarkusio/quarkusio.github.io/pull/2153