DOC: add reshaping visuals to the docs (Reshaping and Pivot Tables)
- [x] closes #20898
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

can you post a rendered version here? Here's a render of the screen up until just after the last figure

![reshaping_viz](https://user-images.githubusercontent.com/1007956/39725429-a2ab1f1a-5200-11e8-9bd6-d03a28ac54b0.png)
 lgtm. @TomAugspurger  Cool, thanks!