templates,docs: leave templates in v0.2 (#27952)

all template installs will now have to declare `--branch v0.2` to make
clear they aren't compatible with langchain 0.3 (most have a pydantic v1
setup). e.g.

```
langchain-cli app add pirate-speak --branch v0.2
```

docs/scripts/arxiv_references.py

@@ -406,7 +406,9 @@ def _format_api_ref_url(doc_path: str, compact: bool = False) -> str:
 
 
 def _format_template_url(template_name: str) -> str:
-    return f"https://{LANGCHAIN_PYTHON_URL}/docs/templates/{template_name}"
+    return (
+        f"https://github.com/langchain-ai/langchain/blob/v0.2/templates/{template_name}"
+    )
 
 
 def _format_cookbook_url(cookbook_name: str) -> str:

docs/vercel.json

@@ -76,7 +76,7 @@
     },
     {
       "source": "/v0.2/docs/templates/:path(.*/?)*",
-      "destination": "https://github.com/langchain-ai/langchain/tree/master/templates/:path*"
+      "destination": "https://github.com/langchain-ai/langchain/tree/v0.2/templates/:path*"
     },
     {
       "source": "/docs/integrations/providers/mlflow_ai_gateway(/?)",