Improve docs on formatting Excel sheets using xlsxwriter
#### Code Sample, a copy-pastable example if possible

N/A

#### Problem description

I'd like to suggest that the documentation about formatting Excel sheets in the I/O tools page here: https://pandas.pydata.org/pandas-docs/stable/user_guide/io.html?highlight=xlsxwriter#style-and-formatting be improved by putting in a reference to the numerous examples provided by the xlsxwriter documentation here: 

https://xlsxwriter.readthedocs.io/pandas_examples.html

I'm willing to do a PR, but I didn't know if there was some reason for not including the external reference on that particular page.

#### Expected Output

N/A

#### Output of ``pd.show_versions()``

pandas 0.24.2
