docs: add runnable code samples for reading methods

[ashleyxuu]

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Fixes #<issue_number_goes_here> 🦕

[ashleyxuu]

Just did the staging: https://cloud.devsite.corp.google.com/python/docs/reference/bigframes/latest/bigframes.session.Session#bigframes_session_Session_read_gbq_table

The question now is to choose (using read_gbq as an example)

1. df = read.gbq("xxx") -> don't display the output
2. read.gbq("xxx").head(5) -> showing the output for first 5 rows

[ashleyxuu]

We decided to follow the pattern like below for adding runnable and testable code samples:

df = read.gbq("xxx")
df.head(2)

The new staging: https://cloud.devsite.corp.google.com/python/docs/reference/bigframes/latest/bigframes.session.Session#bigframes_session_Session_read_csv