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~~

Heh, this pull request is [failing differently](https://github.com/pandas-dev/pandas/pull/38771/checks?check_run_id=1620282550#step:4:67) than https://github.com/pandas-dev/pandas/pull/38735: `flake8-rst` is giving `F821 undefined name 'tips'`. I tried the `:flake8-add-ignore: F821` [Custom Role](https://github.com/kataev/flake8-rst#custom-roles), but that didn't work.

Sigh. Though I planned to use these includes for [the Excel page](https://pandas.pydata.org/pandas-docs/dev/), maybe not worth the trouble.

Thoughts? > Heh, this pull request is failing differently than #38735: flake8-rst is giving F821 undefined name 'tips'. I tried the :flake8-add-ignore: F821 Custom Role, but that didn't work.

Do you have the commit that tried this? Did you add it to the list of extensions in conf.py? Pushed it up in https://github.com/pandas-dev/pandas/pull/38837. Closing in favor of https://github.com/pandas-dev/pandas/pull/38887.