Where to put documentation of the individual extractors?

[MRuecklCC]

Currently the expected extractor behaviors are documented in German in docs/acceptance.md. This is "rather far" (actually not really) away from the implementations increasing the risk of getting out of sync. One other advantage of moving it to the class documentation would be, that we can easily expose the documentation of the individual extractors via the OpenAPI spec. Contrived oversimplified example:

from features.security import Security

@app.get(
    "/security",
   ...
    description=Security.__doc__,
)
def security(...):
   ...

[MRuecklCC]

Related #152