Argument consistency and clarity

[geocoug]

Rename stg_schema to staging_schema for consistency & sync CLI argument descriptions with the help text.

Documentation and help message updates:

• README.md: Updated the help message options to provide more detailed descriptions for each option.
• docs/examples.md: Corrected the parameter name from stg_schema to staging_schema in the example script.

Code updates for consistency and clarity:

• src/pg_upsert/cli.py: Updated the help messages for various CLI options to be more descriptive and consistent. [1] [2] [3]
• src/pg_upsert/upsert.py: Renamed the parameter stg_schema to staging_schema throughout the file for consistency and updated related docstrings and validation messages. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]