docs(bigquery): add Community BigQuery connection guide (#1698) (#2054)

* docs(bigquery): add Community BigQuery connection guide (#1698)

Add docs/guides/bigquery.md and link it from README.md.

Closes #1698

* docs(bigquery): clarify permissions and keyfile paths

---------

Co-authored-by: zgq <openai0229@gmail.com>
Co-authored-by: openai0229 <136558319+openai0229@users.noreply.github.com>