[FEATURE] Update documentation

[IMC07]

Is your feature request related to a problem? Please describe.
Add explanation of the meaning of the configuration settings:
enable_for_source_dq_validation and enable_for_target_dq_validation

The explanation:
SE has three phases:

1. Take the source dataframe and run query_dq and agg_dq on it, when you enable_for_source_dq_validation is true
2. If the first step succeeds, run row_dq
3. Take the dataframe from row_dq and run query_dq and agg_dq on it, when you enable_for_target_dq_validation is true

Would also be nice to have the explanation of all the other configuration settings in place.