DOC: Additional items for the cheat sheet
#### Location of the documentation

https://pandas.pydata.org/Pandas_Cheat_Sheet.pdf

#### Potential Cheat Sheet Improvements

Per discussion at https://github.com/pandas-dev/pandas/pull/39806#pullrequestreview-590105681 , add a third page to the cheat sheet:

- More visualization examples that use pandas plotting (no dependence on third party libraries)
- List of frequently used options shown here: https://pandas.pydata.org/pandas-docs/stable/user_guide/options.html#frequently-used-options
- I/O: A whole section showing a variety of popular IO usage (CSV, Excel, SQL, HTML), and also output file formats (feather, parquet, HDF)
- An Apply Functions section 
- The new Extension Types (String, Integer, Float) and how pd.NA works
- Anything else that could fill up the space (if needed)


