DOC: add dropdown

Author: jklymak

doc/conf.py

@@ -326,7 +326,8 @@ def matplotlib_reduced_latex_scraper(block, block_vars, gallery_conf,
     "switcher": {
         "json_url": "https://matplotlib.org/en/latest/_static/switcher.json",
         "url_template": "https://matplotlib.org/{version}/"
-    }
+    },
+    "navbar_end": ["version-switcher"]
 }
 include_analytics = is_release_build
 if include_analytics: