docs: add link to DataFrames intro to improve SEO

[Adam-Dupaski]

Improving SEO for https://cloud.google.com/bigquery/docs/bigquery-dataframes-introduction - see b/375503333

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Fixes #<issue_number_goes_here> 🦕

[tswast]

Note to others who may review this, some repos templatize README.rst, but bigframes excludes this (https://github.com/googleapis/python-bigquery-dataframes/blob/main/owlbot.py#L49), so we don't have to worry about owlbot overwriting these changes.