docs: update README to point to new summary pages (#402)

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Fixes b/263399076 🦕

Updates the link to the client library reference docs page. The page doesn't exist yet but will be added in a future CL, and mentioned in the TOC from #378.

Author: dandhlee

README.rst

@@ -15,7 +15,7 @@ Documentation
 
 * `BigQuery DataFrames source code (GitHub) <https://github.com/googleapis/python-bigquery-dataframes>`_
 * `BigQuery DataFrames sample notebooks <https://github.com/googleapis/python-bigquery-dataframes/tree/main/notebooks>`_
-* `BigQuery DataFrames API reference <https://cloud.google.com/python/docs/reference/bigframes/latest>`_
+* `BigQuery DataFrames API reference <https://cloud.google.com/python/docs/reference/bigframes/latest/summary_overview>`_
 * `BigQuery documentation <https://cloud.google.com/bigquery/docs/>`_