Update cheatsheet
This PR contributes the following items to the cheatsheet:
- 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
- pd.NA
Will make further additions based on review by maintainers. 
@MichaelTiemannOSC thank you for the feedback and sorry for the delay in getting back! Will address these asap.  This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks for the pull request, but this PR appears to be stale. Additionally it appears https://github.com/pandas-dev/pandas/pull/43036 must be addressed first before other cheat sheet changes so closing.

Happy to have other pull requests with the label `good first issue`