docs: version dropdown (#21784)

docs/docusaurus.config.js

@@ -204,6 +204,21 @@ const config = {
               },
             ]
           },
+          {
+            type: "dropdown",
+            label: "v0.2",
+            position: "right",
+            items: [
+              {
+                label: "v0.2",
+                href: "/docs/introduction"
+              },
+              {
+                label: "v0.1",
+                href: "https://python.langchain.com/v0.1/docs/get_started/introduction"
+              }
+            ]
+          },
           {
             type: "dropdown",
             label: "🦜️🔗",