docs: Update switcher.json for $REF_NAME

Author: hansthen

docs/_static/switcher.json

@@ -4,10 +4,14 @@
     "url": "https://python-visualization.github.io/folium/dev/"
   },
   {
-    "name": "latest (0.20.0)",
-    "version": "0.20.0",
+    "name": "latest (1.0.0-pre.1)",
+    "version": "1.0.0-pre.1",
     "url": "https://python-visualization.github.io/folium/latest/"
   },
+  {
+    "version": "0.20.0",
+    "url": "https://python-visualization.github.io/folium/v0.20.0/"
+  },
   {
     "version": "0.19.7",
     "url": "https://python-visualization.github.io/folium/v0.19.7/"