Docs Update: Mention dunder init requirement for custom compliance

[jeffkala]

Should be some mention in the documentation for custom compliance that the folder with the custom logic needs to be a module, hence requires the __init__.py file.

https://docs.nautobot.com/projects/golden-config/en/latest/user/app_feature_compliancecustom/#example

[itdependsnetworks]

This should be the only requirement:

This is the string representation of the function and must be in Python importable into Nautobot and the workers.

It is already documented. There are likely many things to consider to "be Python importable into Nautobot", but I wouldn't single this one out as it implies we would like all of them.