Marking experimental API elements

[lars-reimann]

Once we start stabilizing the API, we should have a way to mark the remaining experimental API elements. Since Python does not have a built-in mechanism for this, I'd propose that we add the prefix experimental_ to their name. This is obvious and guarantees that there is going to be a breaking change involving this API element in the future. We should document this somewhere.

[lars-reimann]

We can create a custom decorator and integrate it into Griffe. Ideally, the stub generator would also understand it.