Add model docs example

[gsnider2195]

Nautobot expects model docs to exist in docs/models/{cookiecutter.model_class_name.lower()}.md to be able to automatically generate the docs links on some of the generic pages. We should generate this file on bake. As part of this change we can also change the base TestCase for tests/test_models.py to the correct base class of nautobot.apps.testing.ModelTestCases.BaseModelTestCase