docs: add simple query connection read api sample

[PhongChuong]

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Fixes #3404 ☕️

[conventional-commit-lint-gcf]

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[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/com/example/bigquery/SimpleQueryConnectionReadApi.java:19, tag bigquery_simple_query_connection_read_api

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[PhongChuong]

Thanks for the review.