Make documentation version-specific for stable versions

[mabijkerk]

Since we're working with 'frozen' model releases in the stable versions, the documentation should also be made version specific.

The documentation is continuously updated and therefore always up to date with the latest version of the model (or to be precise, it is up to date with beta). If new features have been added, or features have been changed, after a stable version release, and the documentation is updated along with it, then the documentation will be out of synch with the stable version.

@noracato does this capture the general idea?

[aaccensi]

With Docusaurus this should be possible with the following: https://docusaurus.io/docs/versioning