DOC: make shared includes for describing the datasets in the Getting Started tutorials
Just a refactor; the net result should be the same.

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