DOC: add example to insert (#39313)
- [x] closes #39313
- [x] tests added / passed
- [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 - no 

Thanks @nofarm3 - the example looks good, but I get a couple of warnings when trying to build it locally (`python make.py --single pandas.DataFrame.insert`):
```
reading sources... [100%] reference/api/pandas.DataFrame.insert                                                                                                                                           
/home/marco/pandas-marco/pandas/core/frame.py:docstring of pandas.core.frame.DataFrame.insert:58: WARNING: Unexpected indentation.
/home/marco/pandas-marco/pandas/core/frame.py:docstring of pandas.core.frame.DataFrame.insert:59: WARNING: Block quote ends without a blank line; unexpected unindent.
```

 @MarcoGorelli Thanks Marco, fixed.
I think the current failures are not related to my change.