Add docs and example for `SQLThresholdCheckOperator`

[eladkal]

Body

The SQLThresholdCheckOperator isn't mentioned in our docs
https://airflow.apache.org/docs/apache-airflow-providers-common-sql/stable/operators.html

The task is to add information about it with example

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.