Update `docs.yml` once new releases become available

[hcho3]

This section should be updated once new releases of nvForest become available.
https://github.com/rapidsai/docs/blob/1459fbdcdf18820573542290c4f380e21f18599b/_data/docs.yml#L166-L169

• Enable "stable" once 26.06 is released
• Enable "legacy" once 26.08 is released