docs: use restructured text to allow publishing to PyPI

[tswast]

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Fixes #<issue_number_goes_here> 🦕

[tswast]

Presubmit test failure:

_________________ test_bq_session_create_temp_table_clustered __________________
[gw0] linux -- Python 3.12.7 /tmpfs/src/github/python-bigquery-dataframes/.nox/system-3-12/bin/python

bigquery_client = <google.cloud.bigquery.client.Client object at 0x152ebcbca4e0>

    def test_bq_session_create_temp_table_clustered(bigquery_client: bigquery.Client):
        session_resource_manager = bigquery_session.SessionResourceManager(
            bigquery_client, "US"
        )
        cluster_cols = ["string field", "bool field"]
    
        session_table_ref = session_resource_manager.create_temp_table(
            TEST_SCHEMA, cluster_cols=cluster_cols
        )
        session_resource_manager._keep_session_alive()
    
        result_table = bigquery_client.get_table(session_table_ref)
        assert result_table.schema == TEST_SCHEMA
        assert result_table.clustering_fields == cluster_cols
    
        session_resource_manager.close()
>       with pytest.raises(google.api_core.exceptions.NotFound):
E       Failed: DID NOT RAISE <class 'google.api_core.exceptions.NotFound'>

[tests/system/small/test_bq_sessions.py:60](https://cs.corp.google.com/piper///depot/google3/tests/system/small/test_bq_sessions.py?l=60): Failed

Looks like it's happening on main, too.