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)
