API Reference: class methods have no documentation

[Huite]

See for example the MF6 River documentation:

https://deltares.github.io/imod-python/api/generated/mf6/imod.mf6.River.html

The methods have no generated pages associated with them, since we've chosen to specify the generated methods manually. We should probably use a different sphinx directive here to make sure these methods are in fact generated, although they don't have to be available in the index.