DOC: develop a set of standard example DataFrames for use in docstring examples
Related to https://github.com/pandas-dev/pandas/pull/19704. I didn't find an existing open issue, only a discussion mentioning this in https://github.com/pandas-dev/pandas/pull/16520 (@datapythonista it was actually you then! I didn't realize that :-))

I think it would be good to have a set of standard DataFrames that we reuse throughout our docs (to start with in the docstrings, but we could actually also use a standardized set for the user guide):

- Some small, more "realistic" dataframes would make it is easier to reason about than dummy random data + adds familiarity when reading multiple docstrings
- Makes it easier for contributors to add examples to the docstring as they don't have to invent their own data each time

I don't think there will be "one example dataframe to rule them all", but it would be nice to have a set of them that can cover most of the use cases. 
So we can post some ideas here and discuss them, trying to get to a list.

Side question is whether we want to always define them with code in the docstring, or want to have some example data loading capabilities (eg like seaborn, it examples always start with a `iris = sns.load_dataset("iris")` or other dataset). It can also be a mixture of both of course.