Mark APIs in app framework as experimental

[awaelchli]

📚 Documentation

Decide which parts of the app framework are experimental and change rapidly. Mark them as experimental in the docs and or source code.

cc @Borda @rohitgr7 @Borda @tchaton @carmocca

[Borda]

@lantiga do you have a suggestion where and eventually what all classes all be marked so?

[awaelchli]

Example of such changes that happen quickly after the release: #15314
Marking the class there experimental would be good to justify the removals.

[Borda]

shall be done for #15547

[lantiga]

All of app should be marked as experimental, we don't make any commitment to not ship changes in patch releases for this part.

[lantiga]

How would we go about marking?

I'm good with standardized a string / tag in docstring? Or should we reach out to something more involved like a decorator?