DOC: Adding links to the cheat sheets
#### Location of the documentation

Cheat Sheets

#### Documentation problem

It would be great to have web-links to the official pandas docs included in the cheat sheets.

#### Suggested fix for documentation

I already implemented this [here](https://github.com/OliEfr/pandas/blob/master/doc/cheatsheet/Pandas_Cheat_Sheet.pdf). It works well! However, I am not sure how to create a pull request now.
