DOC: create shared includes for comparison docs
This will help ensure consistency between the examples.

Similar to https://github.com/pandas-dev/pandas/pull/38735, but focusing on shared content in a different folder. Particularly interested to see if the build fails in the same way.

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