DOC: Add Notes and Examples back to GroupBy.apply docs template
- [X] xref #37875 (see https://github.com/pandas-dev/pandas/issues/37875#issuecomment-790026815)
- [ ] tests added / passed (n/a)
- [X] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry (n/a)

I think the remaining check failures are unrelated. thanks @TheNeuralBit 

pls check https://pandas.pydata.org/pandas-docs/dev/ after this builds for correctness Thanks @jreback. The [page](https://pandas.pydata.org/pandas-docs/dev/reference/api/pandas.core.groupby.GroupBy.apply.html) LGTM:
![image](https://user-images.githubusercontent.com/675055/109998315-f70a9700-7cc5-11eb-91d0-7e3e3e3fdbc2.png)
