DOC: Include a small preview of the groupby() user guide in its API reference

- [x] closes #43839
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. ok this is fine @tobiasmcnulty if you can merge master and make this not a draft. thanks @tobiasmcnulty 