Translate supervisor tutorial to js (langchain-ai#981)

Just translates the Python version of the supervisor tutorial to js. 

- Depends on langchain-ai/langchainjs#9228
- See also langchain-ai/langchainjs#9235

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Lauren Hirata Singh <lauren@langchain.dev>

Author: 3 people

src/docs.json

@@ -669,8 +669,9 @@
                     "pages": [
                       "oss/javascript/langchain/knowledge-base",
                       "oss/javascript/langchain/rag",
-                      "oss/javascript/langgraph/agentic-rag",
-                      "oss/javascript/langchain/sql-agent"
+                      "oss/javascript/langchain/sql-agent",
+                      "oss/javascript/langchain/supervisor",
+                      "oss/javascript/langgraph/agentic-rag"
                     ]
                   },
                   {