add v0.16.0 to docs switcher (#1921)

Author: Conengmo

docs/_static/switcher.json

@@ -3,6 +3,10 @@
     "version": "latest",
     "url": "https://python-visualization.github.io/folium/latest/"
   },
+  {
+    "version": "v0.16.0",
+    "url": "https://python-visualization.github.io/folium/v0.16.0/"
+  },
   {
     "version": "v0.15.1",
     "url": "https://python-visualization.github.io/folium/v0.15.1/"