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~~
There's also overlap in the data set descriptions between the [combining data](https://pandas.pydata.org/pandas-docs/stable/getting_started/intro_tutorials/08_combine_dataframes.html) and [time series](https://pandas.pydata.org/pandas-docs/stable/getting_started/intro_tutorials/09_timeseries.html) tutorials. I wasn't sure which was correct, so didn't touch those. thanks @afeld. pls check that this renders nicely in the built docs:    https://pandas.pydata.org/pandas-docs/dev/