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~~

Tests pass! 🎉  thanks @afeld 

pls have a look at the built docs and issue a followup PR if needed.

OT: pls comment on your open PRs / merge master for them to see what we need to merge (IIRC some were WIP) Yep yep, they look how I'd expect 👍 

- https://pandas.pydata.org/docs/dev/getting_started/comparison/comparison_with_sas.html
- https://pandas.pydata.org/docs/dev/getting_started/comparison/comparison_with_stata.html
- https://pandas.pydata.org/docs/dev/getting_started/comparison/comparison_with_sql.html