Add an Example DAG / Docs for SQL file support in transform

[kaxil]

Currently, we allow users to pass a SQL file to the sql param in the transform decorator and TransformOperator.

This is handled by the following code:

https://github.com/astronomer/astro-sdk/blob/6b90c92029c7470f5e383345fcdaf77861c8cb94/src/astro/sql/operators/base_decorator.py#L111-L113

We should add an example DAG that uses a SQL file or document it and also mention it in docstring that users can pass a SQL file too.