DOC: show Parquet examples with default engine (without explicit pyarrow/fastparquet engine keyword)
Encountered this in https://github.com/pandas-dev/pandas/pull/61864, but in general for the readability of our doc page, I feel that it is not needed to show every single code example in this section with both pyarrow and fastparquet (certainly because in practice the fastparquet result is then ignored, and we only show the resulting dtypes for the pyarrow one). 
We already mention in the text itself the engine keyword and the different options.


Thanks @jorisvandenbossche 