INTPYTHON-789 Document how to configure contrib apps with models

[timgraham]

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[aclark4life]

I'll add these to the project template, I wonder why we didn't notice sessions which has to be installed for Django to run but currently doesn't have a custom apps.py? I assume it's because it has a model but the model is not a critical part of the app's functionality, but I haven't looked yet.

[timgraham]

The session model uses session_key = models.CharField(_("session key"), max_length=40, primary_key=True) rather than AutoField, so I was wrong to list it here. The other apps aren't enabled in the default project template's INSTALLED_APPS. I suppose they could be added to the template, even if unused.