Purpose

Traditionally, Sphinx's autosummary generation for exceptions is quite succinct: just the class name and the docstring if any. This make sense in general, as Exception subclasses typically do not add attributes and methods. However, in the case of a more complex exception class with additional attributes and methods, users could create a custom autosummary/exception.rst template to render additional content in the Exception subclass.

The second commit in this PR enhances autodoc to populate members/attributes/methods in exceptions, treating them similarly as a class. By default, the template machinery will still render exceptions in a succinct way (through the fallback to the autosummary/base.rst template). However, users can not implement their own autosummary/exception.rst template for a more detailed rendering of their Exception subclasses.