removed __init__ methods from SQLAlchemy models in the documentation because the latter generates a default constructor
changed the explanation of the (now missing) __init__ method
removed MyModel.__init__ from the scaffolds
Add small note showing example usage of MyModel
Update line numbers as appropriate

Removing __init__ moved some code around, update the line numbers as
appropriate so that the right sections of code get highlighted.
Merge pull request #1115 from bertjwregeer/docs/remove_init_sqlalchemy

Docs: remove init sqlalchemy