Support Class Docstrings

[Sam-Armstrong]

Currently, DocPilot only supports adding docstrings to functions; it would be good to also support classes. Doing this will require the following (according to the code at the time this issue was created):

• Adding logic to the script to find the location to put the docstring for a class (which will be different to the logic for finding a function). A new generate_cls_docstring function should probably be created alongside additional logic in add_docstring to achieve this.
• Engineering the existing class prompt (resources/class_prompt.txt) to get the best results