DOC: update `pandas/core/ops.py` docstring template to accept examples
Checklist for the pandas documentation sprint

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] It has been proofread on language by another sprint participant

The Chicago assigned docstrings uses a template with string formatting. To add examples for the various DataFrame operators, I modified the template to allow examples for each operator.

Opening this PR to get comments on this workflow.

I will work with all the ChiPy contributors to make sure we can get their examples in once we figure out the best way forward.
This one is tricky, since we have so many potential examples. I think ideally we would have a docstring stub that creates the dataframes, and then examples stubs for each of the arithmetic methods. Something like (untested)


```python
_arith_frames = """\
>>> a = pd.DataFrame([1, 1, 1, np.nan], index=['a', 'b', 'c', 'd'],
                     columns=['one'])
>>> a
   one
a  1.0
b  1.0
c  1.0
d  NaN
>>> b = pd.DataFrame(dict(one=[1, np.nan, 1, np.nan],
                          two=[np.nan, 2, np.nan, 2]),
                     index=['a', 'b', 'd', 'e'])
>>> b
   one  two
a  1.0  NaN
b  NaN  2.0
d  1.0  NaN
e  NaN  2.0
"""

_add_doc_frame = """\
Examples
--------
{create}
>>> a.add(b, fill_value=0)
   one  two
a  2.0  NaN
b  1.0  2.0
c  1.0  NaN
d  1.0  NaN
e  NaN  2.0 
""".format(create=_arith_frames)
``` @alysivji are you basing all the examples off the same original dataframes? If not, then ignore my comments. @alysivji Do you want us to merge this one? So it is easier for the other PRs to add the examples? @TomAugspurger each team did different examples.

@jorisvandenbossche if we can come up with a best practice i think that would be best. i can help get all the examples in once we figure out the best workflow. @alysivji I think you may know best here, since you've seen some of the other examples I think.

I suspect that the PRs would have many conflicts (removing the old example, updating the kwargs, etc.) and that the only distinct bits are the actual new example and updating the `df_examples` value. If that's the case, let's merge this and then start taking PRs that have an updated copy of master + the new example. Yes, I think it is fine to merge this. thanks @alysivji 