Added links to useful discussions of groupby and SettingWithCopyWarning
Added links to useful discussions of `groupby` and `SettingWithCopyWarning`.

Did not run tests as only docs changed, but `python make.py --single groupby` and `python make.py --single indexing` successfully create the respective html files.

- [x] closes #17505
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
