`Protocol` should be documented as using `ABCMeta` as metaclass

Documentation

Came up in #127648.

Currently the documentation for typing.Protocol doesn't mention that Protocol has ABCMeta in its metaclasses (indirectly via _ProtocolMeta) and that its methods can (should?) be marked with @abstractmethod. In fact, neither the string "ABCMeta" nor "abstractmethod" appear in the documentation for the typing module.

I also suggest to add @abstractmethod to the examples using a Protocol as that is probably a best practice.

Finally, the documentation for Protocol suggests to see PEP 544 for more details. I don't think this is particularly helpful. All information relevant for users should be in the main documentation. PEP 544 should only be suggested as a historical document. (And I don't suggest to directly link to the typing spec, either, although a general link at the top of the page would be useful.)

Beginne mit der im Issue verlinkten Dokumentation zu typing.Protocol und vergleiche ihre Hinweise mit PEP 544. Überprüfe die Protocol-Beispiele und -Erklärungen hinsichtlich der angeforderten Abdeckung von ABCMeta, abstractmethod und Referenzen. Die Aufgabe gilt als erledigt, wenn die relevanten nutzerseitigen Details dokumentiert sind und die PEP-Referenz als historischer Kontext eingeordnet ist.