ArduinoJson 6.0.0-beta was release last week, but the documentation was not there yet.
Well, good news! I finally found the time to update a significant part of the documentation.
arduinojson.org contains both version of the documentation, you can select the target version on the navigation bar on the top of the website.
Still to be done:
At some point, the book will be updated, but not while the version 6 is in beta, which could be long.