DOC: add Comparison with Excel
- [x] closes https://github.com/pandas-dev/pandas/issues/22993
- [x] tests added / passed
- [ ] ~~passes `black pandas`~~
- [ ] ~~passes `git diff upstream/master -u -- "*.py" | flake8 --diff`~~
- [ ] ~~whatsnew entry~~

## Background

I teach [a class on pandas for public policy students](https://github.com/afeld/python-public-policy/blob/master/syllabus.md#readme), and for many of them, spreadsheets are the only point of reference they have for working with tabular data. It would be very helpful to have (official) document comparing the two to point them to.

This is my first contribution to pandas and first time using reStructuredText, so feedback welcome. Thanks in advance!

## TODOs

Making a running checklist to show what I've done already, and what else I plan to do. Hoping for some preliminary feedback (like is there still interest in having this page) before spending too much more time on it.

Happy to continue in this pull request until complete with all of them, or get this merged sooner than later and take care of the others in follow-up pull requests. Slight preference for the latter (some documentation being better than none, less to review at once, etc.), but open to whatever.

- [x] start with what @rotuna did in https://github.com/pandas-dev/pandas/pull/23042
- [x] add links to canonical Excel documentation, as [previously requested](https://github.com/pandas-dev/pandas/pull/23042#discussion_r224289676)
- [x] fix for CI
- [x] link from Getting Started pages
- [ ] incorporate structure from [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
   - [x] Data structures
   - [ ] Data input / output
      - [x] Mention `read_excel()`
   - [ ] Data operations
   - [ ] String processing
   - [ ] Merging
   - [ ] Missing data
   - [ ] GroupBy
   - [ ] Other considerations
- [ ] convert examples to use [`tips` dataset](https://raw.github.com/pandas-dev/pandas/master/pandas/tests/io/data/csv/tips.csv)
- [ ] add info about charting

## Questions

- [x] Which whatsnew file should I add to?
- [x] I noticed that [`doc/source/_static` is in the `.gitignore`](https://github.com/pandas-dev/pandas/blob/54682234e3a3e89e246313bf8f9a53f98b199e7b/.gitignore#L113), but [there are files checked into that folder](https://github.com/pandas-dev/pandas/tree/master/doc/source/_static). Is that intentional? - https://github.com/pandas-dev/pandas/pull/38739
- [ ] Some of the comparison documentation refers to "columns", while other refer to "Series". Is there a preference, or can they be used interchangeably?
- [x] Since spreadsheet software is largely interchangeable/compatible, would it make sense to make the page more general as "Comparison to spreadsheets"?
- [ ] Thoughts about including [slightly more subjective content](https://colab.research.google.com/github/afeld/python-public-policy/blob/main/pandas_crash_course.ipynb#scrollTo=efp6dYYtL-Je), such as _why_ one might want to use spreadsheets vs. pandas?
@afeld can you post a rendered picture here of the new docs page here <details>
<summary>Screenshot of the new page</summary>
<img src="https://user-images.githubusercontent.com/86842/103143847-e3065000-46ec-11eb-94d3-3cfdf8579ca5.png" alt="screenshot of comparison to Excel page">
</details>

Let me know what you think! Nudge on the questions up top. I know there are a lot of commits in here; let me know if you want me to squash.

More I want to do with the page, but hoping this is close to being merge-able. thanks @afeld very nice. built docs will appear here: https://pandas.pydata.org/pandas-docs/dev/ (may take a little for the CI to do it). happy to have a followup as discussed. minor point: https://pandas.pydata.org/pandas-docs/dev/getting_started/comparison/comparison_with_spreadsheets.html#pivot-tables

you can add `margins=True` to have the exact excel pivot table behavior you are showing in the png.