docs: Add forum link to footer (#31795)

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docs/docusaurus.config.js

@@ -253,6 +253,10 @@ const config = {
           {
             title: "Community",
             items: [
+              {
+                label: "LangChain Forum",
+                href: "https://forum.langchain.com/",
+              },
               {
                 label: "Twitter",
                 href: "https://twitter.com/LangChainAI",