Improve documentation to make classes use the "public" module #8894

Discussed in #8893

Currently it seems that sphinx uses a different path than the one of the definition, but it's still not the most common one for some classes.

For example Column is documented as sqlalchemy.schema.Column, while the most common one is sqlalchemy.Column.
In all cases the internal sqlalchemy.sql.schema.Column should not be used in the documentation or users