DOC: clarified read_csv example
* Simple clarification to the read_csv/read_fwf docs so that the example actually loads data into a dataframe.

I wanted to move the User Guide reference down to the examples section similar to the [read_html docs](https://github.com/pandas-dev/pandas/blob/main/pandas/io/html.py#L1095-L1098) but couldn't get it to build correctly locally. This is how it’s done in near every other documentation page for Pandas. I’m disappointed that we can’t merge a disambiguating, net positive change in arguably the most important docs page for pandas. Just an arbitrary page:

https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.sum.html

We are returning the result.

The actual issue here is, that the example is not checked