Add new external resource: Python Graph Gallery

Hi,

I've recently been invited to write this [blogpost](https://matplotlib.org/matplotblog/posts/python-graph-gallery.com/) on the matplotblog page.

After some discussion with the matplotlib twitter account maintainer it sounds like the python graph gallery could be a great fit for your external resources page, so here is the PR.

Please tell me if that looks reasonable to you!

Best,

Author: holtzy

doc/resources/index.rst

@@ -55,6 +55,10 @@ Videos
 Tutorials
 =========
 
+
+* `The Python Graph Gallery <https://www.python-graph-gallery.com>`_
+  by Yan Holtz
+  
 * `Matplotlib tutorial <https://www.labri.fr/perso/nrougier/teaching/matplotlib/>`_
   by Nicolas P. Rougier