docs: Add sample bash script concurrently validating tables

[nj1973]

Description of changes

Adds a simple bash sample script showing users how they can run many small validations concurrently. I've helped customers with this in the past, so this is worth documenting.

Checklist

• I have followed the CONTRIBUTING Guide.
• I have commented my code, particularly in hard-to-understand areas
• I have updated any relevant documentation to reflect my changes, if applicable
• I have added unit and/or integration tests relevant to my change as needed
• I have already checked locally that all unit tests and linting are passing (use the tests/local_check.sh script)
• I have manually executed end-to-end testing (E2E) with the affected databases/engines

[nj1973]

/gcbrun