DOC: Cookbook recipe for emulating R's expand.grid() (#7426)

[onesandzeroes]

closes #7426

As suggested by @jreback in #7426, I've added a quick cookbook recipe for emulating R's expand.grid(), which creates a dataframe from every combination of the values you give it.

None of the existing sections really seemed to apply, so I've added it in a new section.

Example usage:

import itertools
import pandas as pd

def expand_grid(data_dict):
    rows = itertools.product(*data_dict.values())
    return pd.DataFrame.from_records(rows, columns=data_dict.keys())

df = expand_grid(
    {'height': [60, 70],
     'weight': [100, 140, 180],
     'sex': ['Male', 'Female']}
)

df
Out[2]: 
       sex  weight  height
0     Male     100      60
1     Male     100      70
2     Male     140      60
3     Male     140      70
4     Male     180      60
5     Male     180      70
6   Female     100      60
7   Female     100      70
8   Female     140      60
9   Female     140      70
10  Female     180      60
11  Female     180      70

[jreback]

you can remove this line

[jreback]

@jorisvandenbossche ?

[jorisvandenbossche]

Can you add df on a new line? (so the resulting dataframes is also shown in the docs)

[onesandzeroes]

@jreback Alright, I've made the suggested changes and squashed down to a single commit, let me know if there's anything more I'd need to do.

[jorisvandenbossche]

Looking good, thanks!