DOC: move third-party packages to new page

Author: jklymak

doc/contents.rst

@@ -18,7 +18,7 @@ Overview
    faq/index.rst
    api/index.rst
    resources/index.rst
-   thirdpartypackages/index.rst
+   Third-party packages <https://matplotlib.org/mpl-third-party/>
    devel/index.rst
 
 .. only:: html

doc/index.rst

@@ -61,7 +61,7 @@ Matplotlib makes easy things easy and hard things possible.
       Extend
 
       - Explore tailored functionality provided by
-        :doc:`third party packages <thirdpartypackages/index>`
+        `third party packages <https://matplotlib.org/mpl-third-party/>`_
       - Learn more about Matplotlib through the many
         :doc:`external learning resources <resources/index>`
 
@@ -160,7 +160,8 @@ helpers in `.axisartist`.
 Third party packages
 ====================
 
-A large number of :doc:`third party packages <thirdpartypackages/index>`
+A large number of 
+`third party packages <https://matplotlib.org/mpl-third-party/>`_
 extend and build on Matplotlib functionality, including several higher-level
 plotting interfaces (seaborn_, HoloViews_, ggplot_, ...), and a projection
 and mapping toolkit (Cartopy_).