docs(bigquery): improve Client and ClientBuilder docs

[alvarowolfx]

Towards #5844

[gemini-code-assist]

Code Review

This pull request significantly improves the documentation for the BigQuery client and ClientBuilder by adding detailed usage guides, architecture explanations, and runnable doctest examples. Feedback was provided to add a hidden import for futures::StreamExt in the BigQuery example doctest to ensure it compiles successfully.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.06%. Comparing base (210fed0) to head (65bdca7).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6201      +/-   ##
==========================================
- Coverage   96.06%   96.06%   -0.01%     
==========================================
  Files         268      268              
  Lines       67175    67175              
==========================================
- Hits        64534    64530       -4     
- Misses       2641     2645       +4     

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[alvarowolfx]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds comprehensive documentation and usage examples (doctests) to the BigQuery client and ClientBuilder structs. The feedback highlights that the newly added doctests using .next().await on streams will fail to compile because the futures::StreamExt trait is not brought into scope in the hidden setup lines.