Add a value for the `autodoc_class_signature` variable to suppress the constructor signature entirely

The autodoc_class_signature variable is useful to indicate whether or not the constructor signature should be part of the class in documentation.
At present it supports two values: "mixed", to include the signature with the class name, and "separated", to print it separately. I would like to have the ability to suppress the signature entirely, given that the constructor (__init__) can be documented separately, in which case the information is redundant.
(In fact, I never understood why the constructor signature was bundled with the class name. Class and constructor are two distinct entities anyway...)

Start by tracing how the autodoc_class_signature variable handles the existing "mixed" and "separated" values, then inspect any tests covering those options. Done means a new value suppresses the constructor signature entirely while preserving separately documented init output, with coverage for the new behavior.