Add Django models import to Site Settings docs #5278
Makes the implied import of Django's models explicit within the Site Settings documentation. This helps clarify where the fields are coming from (eg that they're not Wagtail-specific).
This doesn't seem overly "significant" (as per https://docs.wagtail.io/en/latest/contributing/committing.html#update-changelog-txt-and-release-notes) but let me know if you'd like me to update the changelog/etc.
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