add mpl-template and plotnine to 3rd party doc (matplotlib#10006)

* add mpl-template and plotnine to 3rd party doc

* fix typo -- squash this [ci skip]

* switch to https for plotnine -- squash this [ci skip]

Author: phobson

doc/_static/mpl_template_example.png

@@ -38,6 +38,16 @@ data processing tools. An example plot from the `Cartopy gallery
 Declarative libraries
 *********************
 
+plotnine
+========
+
+`plotnine <https://plotnine.readthedocs.io/en/stable/>`_ implements a grammar
+of graphics, similar to R's `ggplot2 <http://ggplot2.org/>`_. The grammar allows
+users to compose plots by explicitly mapping data to the visual objects that
+make up the plot.
+
+.. image:: /_static/plotnine.png
+
 ggplot
 ======
 `ggplot <https://github.com/yhat/ggplot>`_ is a port of the R ggplot2 package
@@ -160,6 +170,15 @@ annotation boxes) for Matplotlib.
 Miscellaneous
 *************
 
+mpl-template
+============
+`mpl-template <https://austinorr.github.io/mpl-template/index.html>`_ provides
+a customizable way to add engineering figure elements such as a title block,
+border, and logo.
+
+.. image:: /_static/mpl_template_example.png
+    :height: 330px
+
 adjustText
 ==========
 `adjustText <https://github.com/Phlya/adjustText>`_ is a small library for