Update to_dataframe API Docs to focus on schema use

The API documentation for to_dataframe (https://beam.apache.org/releases/pydoc/2.32.0/apache_beam.dataframe.convert.html#apache_beam.dataframe.convert.to_dataframe) is very sparse. It also focuses on specifying a proxy, rather than relying a schema-aware PCollection as an input. This function is often people's entrypoint into the API, so we should make it very clear how to use it. Let's expand the documentation, and focus on using schemas rather than specifying a proxy.

We should also link to the documentation for to_dataframe in https://beam.apache.org/documentation/dsls/dataframes/overview/#embedding-dataframes-in-a-pipeline