Added Pandas books. (#5188)

* Added Pandas books.

* Added empty line.

* Removed a tutorial from programming books and added it into tutorials.

* Removed single-item category, put Pandas tutorial under python.

free-programming-books.md

@@ -2488,6 +2488,7 @@ Kerridge (PDF) (email address *requested*, not required)
 #### Pandas
 
 * [Learn Pandas](https://bitbucket.org/hrojas/learn-pandas) - Hernan Rojas (0.18.1)
+* [pandas: powerful Python data analysis toolkit](https://pandas.pydata.org/docs/pandas.pdf) - Wes McKinney and the Pandas Development Team (:construction: *in process*) (PDF)
 
 
 #### Pyramid

free-programming-interactive-tutorials-en.md

@@ -272,6 +272,7 @@
 * [Learn Python Step by Step](http://www.techbeamers.com/python-tutorial-step-by-step)
 * [Python for Everybody - Interactive](https://runestone.academy/runestone/books/published/py4e-int/index.html) - Barbara Ericson
 * [Python Koans](https://github.com/gregmalcolm/python_koans)
+* [Python Pandas Tutorial: A Complete Introduction for Beginners](https://www.learndatasci.com/tutorials/python-pandas-tutorial-complete-introduction-for-beginners/) - George McIntire, Brendan Martin, Lauren Washington
 * [Python Tutorial](https://www.w3schools.com/python) - W3Schools