docs: add comprehensive query and read results sample

[AnantShukla-Git-Hub]

Fixes #47

This change adds a comprehensive C++ sample demonstrating how to execute
a BigQuery query using the insert+poll workflow and how to read query
results using futures and callbacks.

The sample shows:

• Executing a query with the insert+poll API
• Attaching a continuation using .then()
• Handling errors from the asynchronous result
• Iterating over rows and fields in the result set

This is intended as a beginner-friendly example and does not modify
any core library code.

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[AnantShukla-Git-Hub]

Hi, just checking if anything is needed from my side.
CLA is signed and required checks are running.
Happy to make changes if required.
Thanks!

[AnantShukla-Git-Hub]

Hi, it looks like all checks are waiting for /gcbrun.
Could a collaborator please trigger the CI?

Thanks! 😊