Added notes on built-in templates for Bootstrap CSS into docs.
Added notes on built-in templates for Bootstrap CSS into docs.

PukeCloud/django-sitetree,PukeCloud/django-sitetree,ikresoft/django-sitetree,RamezIssac/django-sitetree,idlesign/django-sitetree,RamezIssac/django-sitetree,caktus/django-sitetree,caktus/django-sitetree,ikresoft/django-sitetree,idlesign/django-sitetree