Add documentation for BigQuery transfer operators

[nivaldoh]

closes: #27093

Adds documentation for the following Google Cloud transfer operators: BigQueryToBigQueryOperator, BigQueryToGCSOperator, BigQueryToMsSqlOperator, BigQueryToMySqlOperator, GCSToBigQueryOperator

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[nivaldoh]

Migrate the GCSToBigQueryOperator to its own page to maintain the established pattern for transfer operators.

[nivaldoh]

Use example in the docstring as BigQueryToMySqlOperator doesn't have its own system test, and creating one may not be necessary.

[nivaldoh]

@turbaszek @mik-laj Ready for review at your earliest convenience.

[potiuk]

Let's see if CI passes.

[potiuk]

Some static checks are failing - you need to solve and rebase.

[nivaldoh]

Thanks for the review. All points have been addressed.

[potiuk]

docs are failing.

[potiuk]

?

[eladkal]

@nivaldoh can you please address the doc test failures?

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.

[shahar1]

@eladkal Please assign me

[potiuk]

@eladkal Please assign me

You can just take the PR and start working on yours (and make reference to this PR). There is no need to assign it.

[eladkal]

resolved in #29466