Should we create a QDState to avoid having to store Brax state descriptors in the info dictionary?

[felixchalumeau]

This question was raised by @Lookatator after the issue about the behavior descriptors we had a few months ago #12

At the moment, we haven't modified the core, we just updated the play step functions to avoid any problem and added a caveat section in the doc to warn the users. But we should still think about Luca's proposition in the future.