DOC: add more sections to spreadsheet comparison
[Preview (link to PDF on Google Drive)](https://drive.google.com/file/d/1uUyTQyEAX3F6h4EJqKz7Mz4KehGBnAig/view?usp=sharing)

This pull request gets closer to full parity with [SAS](https://pandas.pydata.org/pandas-docs/stable/getting_started/comparison/comparison_with_sas.html)/[STATA](https://pandas.pydata.org/pandas-docs/stable/getting_started/comparison/comparison_with_stata.html) comparison pages by adding the Data Input/Output through Merging sections. It still needs Missing Data and GroupBy, but wanted to get this in while I was at a good stopping place. Each section was done in its own commit, if it's easier to review that way.

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- [x] ~~closes~~ part of https://github.com/pandas-dev/pandas/issues/38990
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] ~~whatsnew entry~~
thanks @afeld love the documentation! keep em coming