feat: add subclasses to children and reference

[dandhlee]

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Unless the issue you're trying to solve is unique to this specific repository,
please file an issue and/or send changes upstream to the original as well.

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The function insert_children_on_class only cared about underlying class methods, attributes and properties, which left out subclasses of a class.

To make that happen, search for the parent class by using datam['module'] and see if there is a parent class that exists, then check that both types are class so we don't accidentally insert anything else when we retrieve classes from app.env.docfx_yaml_classes.get(datam[MODULE]).

When actually inserting the children, we separate whether it's class items we're looking for such as attributes, properties and methods, or if it is a subclass of a class with the nested if condition.

template update to this corresponding PR: googleapis/doc-templates#238

Fixes internally filed issue.

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to README are included in PR

[dandhlee]

Adding this as part of unittest is a bit hard to do because of app.env.docfx_yaml_classes, I'll handle the test case as part of testing with goldens in #44.