DOC: remove use of head() in the comparison docs
This helps to clarify the examples by removing code that isn't relevant. Added a dedicated section to the SAS, SQL, and Stata pages.

This builds on https://github.com/pandas-dev/pandas/pull/38933; ~~will rebase and mark as ready for review once that's merged. In the meantime, the last commit is the one that can be reviewed.~~ Thanks!

- [ ] ~~closes #xxxx~~
- [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~~
