DOC: create shared includes for comparison docs, take III
From original pull request (https://github.com/pandas-dev/pandas/pull/38771):

> This will help ensure consistency between the examples.

- [ ] ~~closes #xxxx~~
- [x] tests added / passed
- [ ] ~~passes `black pandas`~~
- [ ] ~~passes `git diff upstream/master -u -- "*.py" | flake8 --diff`~~
- [ ] ~~whatsnew entry~~
